diff --git a/benches/banking_stage.rs b/benches/banking_stage.rs
index c236346946..94272ec837 100644
--- a/benches/banking_stage.rs
+++ b/benches/banking_stage.rs
@@ -74,7 +74,7 @@ fn bench_banking_stage_multi_accounts(bencher: &mut Bencher) {
         let fund = Transaction::system_move(
             &mint.keypair(),
             tx.account_keys[0],
-            mint_total / txes as i64,
+            mint_total / txes as u64,
             mint.last_id(),
             0,
         );
@@ -175,7 +175,7 @@ fn bench_banking_stage_multi_programs(bencher: &mut Bencher) {
         let fund = Transaction::system_move(
             &mint.keypair(),
             tx.account_keys[0],
-            mint_total / txes as i64,
+            mint_total / txes as u64,
             mint.last_id(),
             0,
         );
diff --git a/programs/bpf/rust/noop/build.sh b/programs/bpf/rust/noop/build.sh
index 4d84de1ee9..77390546dd 100755
--- a/programs/bpf/rust/noop/build.sh
+++ b/programs/bpf/rust/noop/build.sh
@@ -7,4 +7,4 @@ mkdir -p "$OUTDIR"
 # cargo +nightly rustc --release -- -C panic=abort --emit=llvm-ir
 cargo +nightly rustc --release -- -C panic=abort --emit=llvm-bc
 cp "$INTERDIR"/deps/noop_rust-*.bc "$OUTDIR"/noop_rust.bc 
-/usr/local/opt/llvm/bin/llc -march=bpf -filetype=obj -o "$OUTDIR"/noop_rust.o "$OUTDIR"/noop_rust.bc
\ No newline at end of file
+/usr/local/opt/llvm/bin/llc -march=bpf -filetype=obj -o "$OUTDIR"/noop_rust.o "$OUTDIR"/noop_rust.bc
diff --git a/programs/bpf/rust/noop/dump.sh b/programs/bpf/rust/noop/dump.sh
index 47180ced45..5703734ad9 100755
--- a/programs/bpf/rust/noop/dump.sh
+++ b/programs/bpf/rust/noop/dump.sh
@@ -1,3 +1,3 @@
 #!/bin/sh
 
-/usr/local/opt/llvm/bin/llvm-objdump -color -source -disassemble target/release/noop_rust.o
\ No newline at end of file
+/usr/local/opt/llvm/bin/llvm-objdump -color -source -disassemble target/release/noop_rust.o
diff --git a/programs/native/bpf_loader/src/lib.rs b/programs/native/bpf_loader/src/lib.rs
index 30cd225706..e08876fb89 100644
--- a/programs/native/bpf_loader/src/lib.rs
+++ b/programs/native/bpf_loader/src/lib.rs
@@ -70,7 +70,7 @@ fn serialize_parameters(keyed_accounts: &mut [KeyedAccount], data: &[u8]) -> Vec
         .unwrap();
     for info in keyed_accounts.iter_mut() {
         v.write_all(info.key.as_ref()).unwrap();
-        v.write_i64::<LittleEndian>(info.account.tokens).unwrap();
+        v.write_u64::<LittleEndian>(info.account.tokens).unwrap();
         v.write_u64::<LittleEndian>(info.account.userdata.len() as u64)
             .unwrap();
         v.write_all(&info.account.userdata).unwrap();
@@ -87,7 +87,7 @@ fn deserialize_parameters(keyed_accounts: &mut [KeyedAccount], buffer: &[u8]) {
     let mut start = mem::size_of::<u64>();
     for info in keyed_accounts.iter_mut() {
         start += mem::size_of::<Pubkey>(); // skip pubkey
-        info.account.tokens = LittleEndian::read_i64(&buffer[start..]);
+        info.account.tokens = LittleEndian::read_u64(&buffer[start..]);
 
         start += mem::size_of::<u64>() // skip tokens
                   + mem::size_of::<u64>(); // skip length tag
diff --git a/rfcs/0001-smart-contracts-engine.md b/rfcs/0001-smart-contracts-engine.md
index 9a0d68212d..914c3fa06f 100644
--- a/rfcs/0001-smart-contracts-engine.md
+++ b/rfcs/0001-smart-contracts-engine.md
@@ -68,7 +68,7 @@ pub struct Transaction {
     pub last_id: Hash,
 
     /// The number of tokens paid for processing and storage of this transaction.
-    pub fee: i64,
+    pub fee: u64,
 
     /// Keys identifying programs in the instructions vector.
     pub program_ids: Vec<Pubkey>,
@@ -86,7 +86,7 @@ Accounts maintain token state as well as program specific memory.
 /// An Account with userdata that is stored on chain
 pub struct Account {
     /// tokens in the account
-    pub tokens: i64,
+    pub tokens: u64,
     /// user data
     /// A transaction can write to its userdata
     pub userdata: Vec<u8>,
@@ -145,7 +145,7 @@ pub enum SystemProgram {
     /// * space - memory to allocate if greater then zero
     /// * program_id - the program id of the new account
     CreateAccount {
-        tokens: i64,
+        tokens: u64,
         space: u64,
         program_id: Pubkey,
     },
@@ -155,7 +155,7 @@ pub enum SystemProgram {
     /// Move tokens
     /// * Transaction::keys[0] - source
     /// * Transaction::keys[1] - destination
-    Move { tokens: i64 },
+    Move { tokens: u64 },
 }
 ```
 The interface is best described by the `Instruction::userdata` that the user encodes. 
diff --git a/sdk/src/account.rs b/sdk/src/account.rs
index 01ec8fdc50..39354d650b 100644
--- a/sdk/src/account.rs
+++ b/sdk/src/account.rs
@@ -5,7 +5,7 @@ use pubkey::Pubkey;
 #[derive(Serialize, Deserialize, Debug, Clone, Default)]
 pub struct Account {
     /// tokens in the account
-    pub tokens: i64,
+    pub tokens: u64,
     /// user data
     /// A transaction can write to its userdata
     pub userdata: Vec<u8>,
@@ -21,7 +21,7 @@ pub struct Account {
 
 impl Account {
     // TODO do we want to add executable and leader_program_id even though they should always be false/default?
-    pub fn new(tokens: i64, space: usize, program_id: Pubkey) -> Account {
+    pub fn new(tokens: u64, space: usize, program_id: Pubkey) -> Account {
         Account {
             tokens,
             userdata: vec![0u8; space],
diff --git a/src/bank.rs b/src/bank.rs
index 522a5a1d2a..5c69d12ef1 100644
--- a/src/bank.rs
+++ b/src/bank.rs
@@ -33,7 +33,7 @@ use std::sync::atomic::{AtomicUsize, Ordering};
 use std::sync::{Arc, Mutex, RwLock};
 use std::time::Instant;
 use storage_program::StorageProgram;
-use system_program::SystemProgram;
+use system_program::{Error, SystemProgram};
 use system_transaction::SystemTransaction;
 use timing::{duration_as_us, timestamp};
 use token_program;
@@ -80,6 +80,7 @@ pub enum BankError {
     LedgerVerificationFailed,
     /// Contract's transaction token balance does not equal the balance after the transaction
     UnbalancedTransaction(u8),
+
     /// Contract's transactions resulted in an account with a negative balance
     /// The difference from InsufficientFundsForFee is that the transaction was executed by the
     /// contract
@@ -528,8 +529,8 @@ impl Bank {
     /// tick that has achieved finality
     pub fn get_finality_timestamp(
         &self,
-        ticks_and_stakes: &mut [(u64, i64)],
-        supermajority_stake: i64,
+        ticks_and_stakes: &mut [(u64, u64)],
+        supermajority_stake: u64,
     ) -> Option<u64> {
         // Sort by tick height
         ticks_and_stakes.sort_by(|a, b| a.0.cmp(&b.0));
@@ -712,7 +713,7 @@ impl Bank {
         instruction_index: usize,
         tx_program_id: &Pubkey,
         pre_program_id: &Pubkey,
-        pre_tokens: i64,
+        pre_tokens: u64,
         account: &Account,
     ) -> Result<()> {
         // Verify the transaction
@@ -727,9 +728,6 @@ impl Bank {
                 instruction_index as u8,
             ));
         }
-        if account.tokens < 0 {
-            return Err(BankError::ResultWithNegativeTokens(instruction_index as u8));
-        }
         Ok(())
     }
 
@@ -763,7 +761,7 @@ impl Bank {
         let tx_program_id = tx.program_id(instruction_index);
         // TODO: the runtime should be checking read/write access to memory
         // we are trusting the hard coded contracts not to clobber or allocate
-        let pre_total: i64 = program_accounts.iter().map(|a| a.tokens).sum();
+        let pre_total: u64 = program_accounts.iter().map(|a| a.tokens).sum();
         let pre_data: Vec<_> = program_accounts
             .iter_mut()
             .map(|a| (a.program_id, a.tokens))
@@ -783,9 +781,14 @@ impl Bank {
         // Call the contract method
         // It's up to the contract to implement its own rules on moving funds
         if SystemProgram::check_id(&tx_program_id) {
-            if SystemProgram::process_transaction(&tx, instruction_index, program_accounts).is_err()
+            if let Err(err) =
+                SystemProgram::process_transaction(&tx, instruction_index, program_accounts)
             {
-                return Err(BankError::ProgramRuntimeError(instruction_index as u8));
+                let err = match err {
+                    Error::ResultWithNegativeTokens(i) => BankError::ResultWithNegativeTokens(i),
+                    _ => BankError::ProgramRuntimeError(instruction_index as u8),
+                };
+                return Err(err);
             }
         } else if BudgetState::check_id(&tx_program_id) {
             if BudgetState::process_transaction(&tx, instruction_index, program_accounts).is_err() {
@@ -873,7 +876,7 @@ impl Bank {
             }
         }
         // The total sum of all the tokens in all the pages cannot change.
-        let post_total: i64 = program_accounts.iter().map(|a| a.tokens).sum();
+        let post_total: u64 = program_accounts.iter().map(|a| a.tokens).sum();
         if pre_total != post_total {
             Err(BankError::UnbalancedTransaction(instruction_index as u8))
         } else {
@@ -1288,7 +1291,7 @@ impl Bank {
     /// `n` tokens where `last_id` is the last Entry ID observed by the client.
     pub fn transfer(
         &self,
-        n: i64,
+        n: u64,
         keypair: &Keypair,
         to: Pubkey,
         last_id: Hash,
@@ -1298,7 +1301,7 @@ impl Bank {
         self.process_transaction(&tx).map(|_| signature)
     }
 
-    pub fn read_balance(account: &Account) -> i64 {
+    pub fn read_balance(account: &Account) -> u64 {
         if SystemProgram::check_id(&account.program_id) {
             SystemProgram::get_balance(account)
         } else if BudgetState::check_id(&account.program_id) {
@@ -1309,7 +1312,7 @@ impl Bank {
     }
     /// Each contract would need to be able to introspect its own state
     /// this is hard coded to the budget contract language
-    pub fn get_balance(&self, pubkey: &Pubkey) -> i64 {
+    pub fn get_balance(&self, pubkey: &Pubkey) -> u64 {
         self.get_account(pubkey)
             .map(|x| Self::read_balance(&x))
             .unwrap_or(0)
@@ -1317,7 +1320,7 @@ impl Bank {
 
     /// TODO: Need to implement a real staking contract to hold node stake.
     /// Right now this just gets the account balances. See github issue #1655.
-    pub fn get_stake(&self, pubkey: &Pubkey) -> i64 {
+    pub fn get_stake(&self, pubkey: &Pubkey) -> u64 {
         self.get_balance(pubkey)
     }
 
@@ -1482,7 +1485,6 @@ mod tests {
     use hash::hash;
     use jsonrpc_macros::pubsub::{Subscriber, SubscriptionId};
     use ledger;
-    use logger;
     use signature::Keypair;
     use signature::{GenKeys, KeypairUtil};
     use std;
@@ -1611,18 +1613,6 @@ mod tests {
         assert_eq!(bank.get_signature(&t1.last_id, &t1.signature), Some(Ok(())));
     }
 
-    #[test]
-    fn test_negative_tokens() {
-        logger::setup();
-        let mint = Mint::new(1);
-        let pubkey = Keypair::new().pubkey();
-        let bank = Bank::new(&mint);
-        let res = bank.transfer(-1, &mint.keypair(), pubkey, mint.last_id());
-        println!("{:?}", bank.get_account(&pubkey));
-        assert_matches!(res, Err(BankError::ResultWithNegativeTokens(0)));
-        assert_eq!(bank.transaction_count(), 0);
-    }
-
     // TODO: This test demonstrates that fees are not paid when a program fails.
     // See github issue 1157 (https://github.com/solana-labs/solana/issues/1157)
     #[test]
@@ -1890,7 +1880,7 @@ mod tests {
         let dummy_leader_id = Keypair::new().pubkey();
         let dummy_leader_tokens = 1;
         let mint = Mint::new_with_leader(
-            length as i64 + 1 + dummy_leader_tokens,
+            length as u64 + 1 + dummy_leader_tokens,
             dummy_leader_id,
             dummy_leader_tokens,
         );
diff --git a/src/bin/bench-tps.rs b/src/bin/bench-tps.rs
index c2bf7009d1..70a6ccb3bf 100644
--- a/src/bin/bench-tps.rs
+++ b/src/bin/bench-tps.rs
@@ -46,7 +46,7 @@ pub struct NodeStats {
     pub tx: u64,  // Total transactions reported by this node
 }
 
-fn metrics_submit_token_balance(token_balance: i64) {
+fn metrics_submit_token_balance(token_balance: u64) {
     println!("Token balance: {}", token_balance);
     metrics::submit(
         influxdb::Point::new("bench-tps")
@@ -313,14 +313,14 @@ fn verify_transfer(client: &mut ThinClient, tx: &Transaction) -> bool {
 /// fund the dests keys by spending all of the source keys into MAX_SPENDS_PER_TX
 /// on every iteration.  This allows us to replay the transfers because the source is either empty,
 /// or full
-fn fund_keys(client: &mut ThinClient, source: &Keypair, dests: &[Keypair], tokens: i64) {
-    let total = tokens * dests.len() as i64;
-    let mut funded: Vec<(&Keypair, i64)> = vec![(source, total)];
+fn fund_keys(client: &mut ThinClient, source: &Keypair, dests: &[Keypair], tokens: u64) {
+    let total = tokens * dests.len() as u64;
+    let mut funded: Vec<(&Keypair, u64)> = vec![(source, total)];
     let mut notfunded: Vec<&Keypair> = dests.iter().collect();
 
     println!("funding keys {}", dests.len());
     while !notfunded.is_empty() {
-        let mut new_funded: Vec<(&Keypair, i64)> = vec![];
+        let mut new_funded: Vec<(&Keypair, u64)> = vec![];
         let mut to_fund = vec![];
         println!("creating from... {}", funded.len());
         for f in &mut funded {
@@ -329,7 +329,7 @@ fn fund_keys(client: &mut ThinClient, source: &Keypair, dests: &[Keypair], token
                 break;
             }
             let start = notfunded.len() - max_units;
-            let per_unit = f.1 / (max_units as i64);
+            let per_unit = f.1 / (max_units as u64);
             let moves: Vec<_> = notfunded[start..]
                 .iter()
                 .map(|k| (k.pubkey(), per_unit))
@@ -376,7 +376,7 @@ fn fund_keys(client: &mut ThinClient, source: &Keypair, dests: &[Keypair], token
     }
 }
 
-fn airdrop_tokens(client: &mut ThinClient, leader: &NodeInfo, id: &Keypair, tx_count: i64) {
+fn airdrop_tokens(client: &mut ThinClient, leader: &NodeInfo, id: &Keypair, tx_count: u64) {
     let mut drone_addr = leader.contact_info.tpu;
     drone_addr.set_port(DRONE_PORT);
 
@@ -494,7 +494,7 @@ fn compute_and_report_stats(
 // First transfer 3/4 of the tokens to the dest accounts
 // then ping-pong 1/4 of the tokens back to the other account
 // this leaves 1/4 token buffer in each account
-fn should_switch_directions(num_tokens_per_account: i64, i: i64) -> bool {
+fn should_switch_directions(num_tokens_per_account: u64, i: u64) -> bool {
     i % (num_tokens_per_account / 4) == 0 && (i >= (3 * num_tokens_per_account) / 4)
 }
 
@@ -653,7 +653,7 @@ fn main() {
         total_keys += target;
         target /= MAX_SPENDS_PER_TX;
     }
-    let gen_keypairs = rnd.gen_n_keypairs(total_keys as i64);
+    let gen_keypairs = rnd.gen_n_keypairs(total_keys as u64);
     let barrier_id = rnd.gen_n_keypairs(1).pop().unwrap();
 
     println!("Get tokens...");
@@ -667,7 +667,7 @@ fn main() {
 
     if num_tokens_per_account > keypair0_balance {
         let extra = num_tokens_per_account - keypair0_balance;
-        let total = extra * (gen_keypairs.len() as i64);
+        let total = extra * (gen_keypairs.len() as u64);
         airdrop_tokens(&mut client, &leader, &id, total);
         println!("adding more tokens {}", extra);
         fund_keys(&mut client, &id, &gen_keypairs, extra);
@@ -730,7 +730,7 @@ fn main() {
     let mut reclaim_tokens_back_to_source_account = false;
     let mut i = keypair0_balance;
     while start.elapsed() < duration {
-        let balance = client.poll_get_balance(&id.pubkey()).unwrap_or(-1);
+        let balance = client.poll_get_balance(&id.pubkey()).unwrap_or(0);
         metrics_submit_token_balance(balance);
 
         // ping-pong between source and destination accounts for each loop iteration
@@ -782,7 +782,7 @@ fn main() {
         }
     }
 
-    let balance = client.poll_get_balance(&id.pubkey()).unwrap_or(-1);
+    let balance = client.poll_get_balance(&id.pubkey()).unwrap_or(0);
     metrics_submit_token_balance(balance);
 
     compute_and_report_stats(
diff --git a/src/bin/genesis.rs b/src/bin/genesis.rs
index dadea25ce1..a60dff0e70 100644
--- a/src/bin/genesis.rs
+++ b/src/bin/genesis.rs
@@ -59,7 +59,7 @@ fn main() -> Result<(), Box<error::Error>> {
     let leader_keypair = leader_config.keypair();
 
     // Parse the input mint configuration
-    let num_tokens = value_t_or_exit!(matches, "num_tokens", i64);
+    let num_tokens = value_t_or_exit!(matches, "num_tokens", u64);
     let file = File::open(Path::new(&matches.value_of("mint").unwrap())).unwrap();
     let pkcs8: Vec<u8> = serde_json::from_reader(&file)?;
     let mint = Mint::new_with_pkcs8(num_tokens, pkcs8, leader_keypair.pubkey(), 1);
diff --git a/src/budget_expr.rs b/src/budget_expr.rs
index dd47847f28..ef1a1fbebd 100644
--- a/src/budget_expr.rs
+++ b/src/budget_expr.rs
@@ -51,17 +51,17 @@ pub enum BudgetExpr {
 
 impl BudgetExpr {
     /// Create the simplest budget - one that pays `tokens` to Pubkey.
-    pub fn new_payment(tokens: i64, to: Pubkey) -> Self {
+    pub fn new_payment(tokens: u64, to: Pubkey) -> Self {
         BudgetExpr::Pay(Payment { tokens, to })
     }
 
     /// Create a budget that pays `tokens` to `to` after being witnessed by `from`.
-    pub fn new_authorized_payment(from: Pubkey, tokens: i64, to: Pubkey) -> Self {
+    pub fn new_authorized_payment(from: Pubkey, tokens: u64, to: Pubkey) -> Self {
         BudgetExpr::After(Condition::Signature(from), Payment { tokens, to })
     }
 
     /// Create a budget that pays tokens` to `to` after being witnessed by 2x `from`s
-    pub fn new_2_2_multisig_payment(from0: Pubkey, from1: Pubkey, tokens: i64, to: Pubkey) -> Self {
+    pub fn new_2_2_multisig_payment(from0: Pubkey, from1: Pubkey, tokens: u64, to: Pubkey) -> Self {
         BudgetExpr::And(
             Condition::Signature(from0),
             Condition::Signature(from1),
@@ -70,7 +70,7 @@ impl BudgetExpr {
     }
 
     /// Create a budget that pays `tokens` to `to` after the given DateTime.
-    pub fn new_future_payment(dt: DateTime<Utc>, from: Pubkey, tokens: i64, to: Pubkey) -> Self {
+    pub fn new_future_payment(dt: DateTime<Utc>, from: Pubkey, tokens: u64, to: Pubkey) -> Self {
         BudgetExpr::After(Condition::Timestamp(dt, from), Payment { tokens, to })
     }
 
@@ -79,7 +79,7 @@ impl BudgetExpr {
     pub fn new_cancelable_future_payment(
         dt: DateTime<Utc>,
         from: Pubkey,
-        tokens: i64,
+        tokens: u64,
         to: Pubkey,
     ) -> Self {
         BudgetExpr::Or(
@@ -97,7 +97,7 @@ impl BudgetExpr {
     }
 
     /// Return true if the budget spends exactly `spendable_tokens`.
-    pub fn verify(&self, spendable_tokens: i64) -> bool {
+    pub fn verify(&self, spendable_tokens: u64) -> bool {
         match self {
             BudgetExpr::Pay(payment)
             | BudgetExpr::After(_, payment)
diff --git a/src/budget_instruction.rs b/src/budget_instruction.rs
index d80d5d2e94..78ca94e2ac 100644
--- a/src/budget_instruction.rs
+++ b/src/budget_instruction.rs
@@ -5,7 +5,7 @@ use chrono::prelude::{DateTime, Utc};
 #[derive(Serialize, Deserialize, Debug, PartialEq, Eq, Clone)]
 pub struct Contract {
     /// The number of tokens allocated to the `BudgetExpr` and any transaction fees.
-    pub tokens: i64,
+    pub tokens: u64,
     pub budget_expr: BudgetExpr,
 }
 
diff --git a/src/budget_program.rs b/src/budget_program.rs
index 8975f2f02b..718c5abc43 100644
--- a/src/budget_program.rs
+++ b/src/budget_program.rs
@@ -235,7 +235,7 @@ impl BudgetState {
     }
 
     //TODO the contract needs to provide a "get_balance" introspection call of the userdata
-    pub fn get_balance(account: &Account) -> i64 {
+    pub fn get_balance(account: &Account) -> u64 {
         if let Ok(state) = deserialize(&account.userdata) {
             let state: BudgetState = state;
             if state.is_pending() {
diff --git a/src/budget_transaction.rs b/src/budget_transaction.rs
index c8eac3ca84..2845119ec4 100644
--- a/src/budget_transaction.rs
+++ b/src/budget_transaction.rs
@@ -16,12 +16,12 @@ pub trait BudgetTransaction {
     fn budget_new_taxed(
         from_keypair: &Keypair,
         to: Pubkey,
-        tokens: i64,
-        fee: i64,
+        tokens: u64,
+        fee: u64,
         last_id: Hash,
     ) -> Self;
 
-    fn budget_new(from_keypair: &Keypair, to: Pubkey, tokens: i64, last_id: Hash) -> Self;
+    fn budget_new(from_keypair: &Keypair, to: Pubkey, tokens: u64, last_id: Hash) -> Self;
 
     fn budget_new_timestamp(
         from_keypair: &Keypair,
@@ -45,7 +45,7 @@ pub trait BudgetTransaction {
         dt: DateTime<Utc>,
         dt_pubkey: Pubkey,
         cancelable: Option<Pubkey>,
-        tokens: i64,
+        tokens: u64,
         last_id: Hash,
     ) -> Self;
 
@@ -55,7 +55,7 @@ pub trait BudgetTransaction {
         contract: Pubkey,
         witness: Pubkey,
         cancelable: Option<Pubkey>,
-        tokens: i64,
+        tokens: u64,
         last_id: Hash,
     ) -> Self;
 
@@ -70,8 +70,8 @@ impl BudgetTransaction for Transaction {
     fn budget_new_taxed(
         from_keypair: &Keypair,
         to: Pubkey,
-        tokens: i64,
-        fee: i64,
+        tokens: u64,
+        fee: u64,
         last_id: Hash,
     ) -> Self {
         let contract = Keypair::new().pubkey();
@@ -106,7 +106,7 @@ impl BudgetTransaction for Transaction {
     }
 
     /// Create and sign a new Transaction. Used for unit-testing.
-    fn budget_new(from_keypair: &Keypair, to: Pubkey, tokens: i64, last_id: Hash) -> Self {
+    fn budget_new(from_keypair: &Keypair, to: Pubkey, tokens: u64, last_id: Hash) -> Self {
         Self::budget_new_taxed(from_keypair, to, tokens, 0, last_id)
     }
 
@@ -157,7 +157,7 @@ impl BudgetTransaction for Transaction {
         dt: DateTime<Utc>,
         dt_pubkey: Pubkey,
         cancelable: Option<Pubkey>,
-        tokens: i64,
+        tokens: u64,
         last_id: Hash,
     ) -> Self {
         let expr = if let Some(from) = cancelable {
@@ -186,7 +186,7 @@ impl BudgetTransaction for Transaction {
         contract: Pubkey,
         witness: Pubkey,
         cancelable: Option<Pubkey>,
-        tokens: i64,
+        tokens: u64,
         last_id: Hash,
     ) -> Self {
         let expr = if let Some(from) = cancelable {
@@ -221,7 +221,7 @@ impl BudgetTransaction for Transaction {
     fn verify_plan(&self) -> bool {
         if let Some(SystemProgram::Move { tokens }) = self.system_instruction(0) {
             if let Some(Instruction::NewBudget(expr)) = self.instruction(1) {
-                if !(self.fee >= 0 && self.fee <= tokens && expr.verify(tokens - self.fee)) {
+                if !(self.fee <= tokens && expr.verify(tokens - self.fee)) {
                     return false;
                 }
             }
@@ -261,8 +261,6 @@ mod tests {
         let keypair0 = Keypair::new();
         let pubkey1 = Keypair::new().pubkey();
         assert!(Transaction::budget_new_taxed(&keypair0, pubkey1, 1, 1, zero).verify_plan());
-        assert!(!Transaction::budget_new_taxed(&keypair0, pubkey1, 1, 2, zero).verify_plan());
-        assert!(!Transaction::budget_new_taxed(&keypair0, pubkey1, 1, -1, zero).verify_plan());
     }
 
     #[test]
diff --git a/src/compute_leader_finality_service.rs b/src/compute_leader_finality_service.rs
index a2842a6c81..00df142283 100644
--- a/src/compute_leader_finality_service.rs
+++ b/src/compute_leader_finality_service.rs
@@ -34,7 +34,7 @@ impl ComputeLeaderFinalityService {
     ) -> result::Result<u64, FinalityError> {
         let mut total_stake = 0;
 
-        let mut ticks_and_stakes: Vec<(u64, i64)> = {
+        let mut ticks_and_stakes: Vec<(u64, u64)> = {
             let bank_accounts = bank.accounts.read().unwrap();
             // TODO: Doesn't account for duplicates since a single validator could potentially register
             // multiple vote accounts. Once that is no longer possible (see the TODO in vote_program.rs,
diff --git a/src/drone.rs b/src/drone.rs
index 0b9cf1288d..4f74dd8375 100644
--- a/src/drone.rs
+++ b/src/drone.rs
@@ -131,7 +131,7 @@ impl Drone {
                 Transaction::system_new(
                     &self.mint_keypair,
                     client_pubkey,
-                    airdrop_request_amount as i64,
+                    airdrop_request_amount as u64,
                     last_id,
                 )
             }
@@ -310,8 +310,8 @@ mod tests {
     #[test]
     #[ignore]
     fn test_send_airdrop() {
-        const SMALL_BATCH: i64 = 50;
-        const TPS_BATCH: i64 = 5_000_000;
+        const SMALL_BATCH: u64 = 50;
+        const TPS_BATCH: u64 = 5_000_000;
 
         logger::setup();
         let leader_keypair = Arc::new(Keypair::new());
diff --git a/src/leader_scheduler.rs b/src/leader_scheduler.rs
index 5ecd5277ec..5d79d032a7 100644
--- a/src/leader_scheduler.rs
+++ b/src/leader_scheduler.rs
@@ -549,7 +549,7 @@ mod tests {
         let num_vote_account_tokens = 1;
         let mint = Mint::new(
             (((num_validators + 1) / 2) * (num_validators + 1)
-                + num_vote_account_tokens * num_validators) as i64,
+                + num_vote_account_tokens * num_validators) as u64,
         );
         let bank = Bank::new(&mint);
         let mut validators = vec![];
@@ -564,7 +564,7 @@ mod tests {
             validators.push(new_pubkey);
             // Give the validator some tokens
             bank.transfer(
-                (i + 1 + num_vote_account_tokens) as i64,
+                (i + 1 + num_vote_account_tokens) as u64,
                 &mint.keypair(),
                 new_pubkey,
                 last_id,
@@ -574,7 +574,7 @@ mod tests {
             let new_vote_account = create_vote_account(
                 &new_validator,
                 &bank,
-                num_vote_account_tokens as i64,
+                num_vote_account_tokens as u64,
                 mint.last_id(),
             ).unwrap();
             // Vote to make the validator part of the active set for the entire test
@@ -743,7 +743,7 @@ mod tests {
     fn test_rank_active_set() {
         let num_validators: usize = 101;
         // Give mint sum(1..num_validators) tokens
-        let mint = Mint::new((((num_validators + 1) / 2) * (num_validators + 1)) as i64);
+        let mint = Mint::new((((num_validators + 1) / 2) * (num_validators + 1)) as u64);
         let bank = Bank::new(&mint);
         let mut validators = vec![];
         let last_id = mint
@@ -756,7 +756,7 @@ mod tests {
             let new_pubkey = new_validator.pubkey();
             validators.push(new_validator);
             bank.transfer(
-                (num_validators - i) as i64,
+                (num_validators - i) as u64,
                 &mint.keypair(),
                 new_pubkey,
                 last_id,
@@ -782,7 +782,7 @@ mod tests {
             let new_pubkey = new_validator.pubkey();
             new_validators.push(new_validator);
             bank.transfer(
-                (num_validators - i) as i64,
+                (num_validators - i) as u64,
                 &validators[i],
                 new_pubkey,
                 last_id,
@@ -803,7 +803,7 @@ mod tests {
         }
 
         // Break ties between validators with the same balances using public key
-        let mint = Mint::new(num_validators as i64);
+        let mint = Mint::new(num_validators as u64);
         let bank = Bank::new(&mint);
         let mut tied_validators_pk = vec![];
         let last_id = mint
@@ -956,7 +956,7 @@ mod tests {
         // Create the bank and validators
         let mint = Mint::new(
             ((((num_validators + 1) / 2) * (num_validators + 1))
-                + (num_vote_account_tokens * num_validators)) as i64,
+                + (num_vote_account_tokens * num_validators)) as u64,
         );
         let bank = Bank::new(&mint);
         let mut validators = vec![];
@@ -971,7 +971,7 @@ mod tests {
             validators.push(new_pubkey);
             // Give the validator some tokens
             bank.transfer(
-                (i + 1 + num_vote_account_tokens) as i64,
+                (i + 1 + num_vote_account_tokens) as u64,
                 &mint.keypair(),
                 new_pubkey,
                 last_id,
@@ -981,7 +981,7 @@ mod tests {
             let new_vote_account = create_vote_account(
                 &new_validator,
                 &bank,
-                num_vote_account_tokens as i64,
+                num_vote_account_tokens as u64,
                 mint.last_id(),
             ).unwrap();
 
diff --git a/src/ledger.rs b/src/ledger.rs
index 79774c5912..a81ed40e07 100644
--- a/src/ledger.rs
+++ b/src/ledger.rs
@@ -623,9 +623,9 @@ pub fn create_tmp_ledger_with_mint(name: &str, mint: &Mint) -> String {
 
 pub fn create_tmp_genesis(
     name: &str,
-    num: i64,
+    num: u64,
     bootstrap_leader_id: Pubkey,
-    bootstrap_leader_tokens: i64,
+    bootstrap_leader_tokens: u64,
 ) -> (Mint, String) {
     let mint = Mint::new_with_leader(num, bootstrap_leader_id, bootstrap_leader_tokens);
     let path = create_tmp_ledger_with_mint(name, &mint);
@@ -646,10 +646,10 @@ pub fn create_ticks(num_ticks: usize, mut hash: Hash) -> Vec<Entry> {
 
 pub fn create_tmp_sample_ledger(
     name: &str,
-    num_tokens: i64,
+    num_tokens: u64,
     num_ending_ticks: usize,
     bootstrap_leader_id: Pubkey,
-    bootstrap_leader_tokens: i64,
+    bootstrap_leader_tokens: u64,
 ) -> (Mint, String, Vec<Entry>) {
     let mint = Mint::new_with_leader(num_tokens, bootstrap_leader_id, bootstrap_leader_tokens);
     let path = get_tmp_ledger_path(name);
diff --git a/src/loader_transaction.rs b/src/loader_transaction.rs
index bfdd06498f..ac15ef5e9c 100644
--- a/src/loader_transaction.rs
+++ b/src/loader_transaction.rs
@@ -14,10 +14,10 @@ pub trait LoaderTransaction {
         offset: u32,
         bytes: Vec<u8>,
         last_id: Hash,
-        fee: i64,
+        fee: u64,
     ) -> Self;
 
-    fn finalize(from_keypair: &Keypair, loader: Pubkey, last_id: Hash, fee: i64) -> Self;
+    fn finalize(from_keypair: &Keypair, loader: Pubkey, last_id: Hash, fee: u64) -> Self;
 }
 
 impl LoaderTransaction for Transaction {
@@ -27,7 +27,7 @@ impl LoaderTransaction for Transaction {
         offset: u32,
         bytes: Vec<u8>,
         last_id: Hash,
-        fee: i64,
+        fee: u64,
     ) -> Self {
         trace!(
             "LoaderTransaction::Write() program {:?} offset {} length {}",
@@ -40,7 +40,7 @@ impl LoaderTransaction for Transaction {
         Transaction::new(from_keypair, &[], loader, userdata, last_id, fee)
     }
 
-    fn finalize(from_keypair: &Keypair, loader: Pubkey, last_id: Hash, fee: i64) -> Self {
+    fn finalize(from_keypair: &Keypair, loader: Pubkey, last_id: Hash, fee: u64) -> Self {
         trace!(
             "LoaderTransaction::Finalize() program {:?}",
             from_keypair.pubkey(),
diff --git a/src/mint.rs b/src/mint.rs
index 70322e111c..e14fac16b4 100644
--- a/src/mint.rs
+++ b/src/mint.rs
@@ -13,17 +13,17 @@ use untrusted::Input;
 pub struct Mint {
     pub pkcs8: Vec<u8>,
     pubkey: Pubkey,
-    pub tokens: i64,
+    pub tokens: u64,
     pub bootstrap_leader_id: Pubkey,
-    pub bootstrap_leader_tokens: i64,
+    pub bootstrap_leader_tokens: u64,
 }
 
 impl Mint {
     pub fn new_with_pkcs8(
-        tokens: i64,
+        tokens: u64,
         pkcs8: Vec<u8>,
         bootstrap_leader_id: Pubkey,
-        bootstrap_leader_tokens: i64,
+        bootstrap_leader_tokens: u64,
     ) -> Self {
         let keypair =
             Keypair::from_pkcs8(Input::from(&pkcs8)).expect("from_pkcs8 in mint pub fn new");
@@ -38,9 +38,9 @@ impl Mint {
     }
 
     pub fn new_with_leader(
-        tokens: i64,
+        tokens: u64,
         bootstrap_leader: Pubkey,
-        bootstrap_leader_tokens: i64,
+        bootstrap_leader_tokens: u64,
     ) -> Self {
         let rnd = SystemRandom::new();
         let pkcs8 = Keypair::generate_pkcs8(&rnd)
@@ -49,7 +49,7 @@ impl Mint {
         Self::new_with_pkcs8(tokens, pkcs8, bootstrap_leader, bootstrap_leader_tokens)
     }
 
-    pub fn new(tokens: i64) -> Self {
+    pub fn new(tokens: u64) -> Self {
         let rnd = SystemRandom::new();
         let pkcs8 = Keypair::generate_pkcs8(&rnd)
             .expect("generate_pkcs8 in mint pub fn new")
diff --git a/src/payment_plan.rs b/src/payment_plan.rs
index 5450bed5cb..c0f5f1e651 100644
--- a/src/payment_plan.rs
+++ b/src/payment_plan.rs
@@ -20,7 +20,7 @@ pub enum Witness {
 #[derive(Serialize, Deserialize, Debug, PartialEq, Eq, Clone)]
 pub struct Payment {
     /// Amount to be paid.
-    pub tokens: i64,
+    pub tokens: u64,
 
     /// The `Pubkey` that `tokens` should be paid to.
     pub to: Pubkey,
diff --git a/src/rpc.rs b/src/rpc.rs
index dccf3af7eb..54063cc6bb 100644
--- a/src/rpc.rs
+++ b/src/rpc.rs
@@ -135,7 +135,7 @@ build_rpc_trait! {
         fn get_account_info(&self, Self::Metadata, String) -> Result<Account>;
 
         #[rpc(meta, name = "getBalance")]
-        fn get_balance(&self, Self::Metadata, String) -> Result<i64>;
+        fn get_balance(&self, Self::Metadata, String) -> Result<u64>;
 
         #[rpc(meta, name = "getFinality")]
         fn get_finality(&self, Self::Metadata) -> Result<usize>;
@@ -172,7 +172,7 @@ impl RpcSol for RpcSolImpl {
         let pubkey = verify_pubkey(id)?;
         meta.request_processor.get_account_info(pubkey)
     }
-    fn get_balance(&self, meta: Self::Metadata, id: String) -> Result<i64> {
+    fn get_balance(&self, meta: Self::Metadata, id: String) -> Result<u64> {
         info!("get_balance rpc request received: {:?}", id);
         let pubkey = verify_pubkey(id)?;
         meta.request_processor.get_balance(pubkey)
@@ -276,7 +276,7 @@ impl JsonRpcRequestProcessor {
             .get_account(&pubkey)
             .ok_or_else(Error::invalid_request)
     }
-    fn get_balance(&self, pubkey: Pubkey) -> Result<i64> {
+    fn get_balance(&self, pubkey: Pubkey) -> Result<u64> {
         let val = self.bank.get_balance(&pubkey);
         Ok(val)
     }
diff --git a/src/rpc_request.rs b/src/rpc_request.rs
index 5ccfdd126d..1d2260d936 100644
--- a/src/rpc_request.rs
+++ b/src/rpc_request.rs
@@ -175,7 +175,7 @@ mod tests {
             Some(json!("deadbeefXjn8o3yroDHxUtKsZZgoy4GPkPPXfouKNHhx")),
         );
         assert!(balance.is_ok());
-        assert_eq!(balance.unwrap().as_i64().unwrap(), 50);
+        assert_eq!(balance.unwrap().as_u64().unwrap(), 50);
 
         let last_id = RpcRequest::GetLastId.make_rpc_request(&rpc_addr, 2, None);
         assert!(last_id.is_ok());
diff --git a/src/signature.rs b/src/signature.rs
index cdc30853e4..17c24b345d 100644
--- a/src/signature.rs
+++ b/src/signature.rs
@@ -84,11 +84,11 @@ impl GenKeys {
         seed
     }
 
-    fn gen_n_seeds(&mut self, n: i64) -> Vec<[u8; 32]> {
+    fn gen_n_seeds(&mut self, n: u64) -> Vec<[u8; 32]> {
         (0..n).map(|_| self.gen_seed()).collect()
     }
 
-    pub fn gen_n_keypairs(&mut self, n: i64) -> Vec<Keypair> {
+    pub fn gen_n_keypairs(&mut self, n: u64) -> Vec<Keypair> {
         self.gen_n_seeds(n)
             .into_par_iter()
             .map(|seed| Keypair::from_seed_unchecked(Input::from(&seed)).unwrap())
@@ -124,7 +124,7 @@ mod tests {
         }
     }
 
-    fn gen_n_pubkeys(seed: [u8; 32], n: i64) -> HashSet<Pubkey> {
+    fn gen_n_pubkeys(seed: [u8; 32], n: u64) -> HashSet<Pubkey> {
         GenKeys::new(seed)
             .gen_n_keypairs(n)
             .into_iter()
diff --git a/src/storage_program.rs b/src/storage_program.rs
index 7010f0d9d1..7c2050d635 100644
--- a/src/storage_program.rs
+++ b/src/storage_program.rs
@@ -31,7 +31,7 @@ impl StorageProgram {
         Pubkey::new(&STORAGE_PROGRAM_ID)
     }
 
-    pub fn get_balance(account: &Account) -> i64 {
+    pub fn get_balance(account: &Account) -> u64 {
         account.tokens
     }
 
diff --git a/src/system_program.rs b/src/system_program.rs
index 0fc5fcee5f..13c4410324 100644
--- a/src/system_program.rs
+++ b/src/system_program.rs
@@ -11,6 +11,7 @@ pub enum Error {
     InvalidArgument,
     AssignOfUnownedAccount,
     AccountNotFinalized,
+    ResultWithNegativeTokens(u8),
 }
 impl std::fmt::Display for Error {
     fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
@@ -30,7 +31,7 @@ pub enum SystemProgram {
     /// * space - memory to allocate if greater then zero
     /// * program_id - the program id of the new account
     CreateAccount {
-        tokens: i64,
+        tokens: u64,
         space: u64,
         program_id: Pubkey,
     },
@@ -40,7 +41,7 @@ pub enum SystemProgram {
     /// Move tokens
     /// * Transaction::keys[0] - source
     /// * Transaction::keys[1] - destination
-    Move { tokens: i64 },
+    Move { tokens: u64 },
 
     /// Spawn a new program from an account
     Spawn,
@@ -56,7 +57,7 @@ impl SystemProgram {
     pub fn id() -> Pubkey {
         Pubkey::new(&SYSTEM_PROGRAM_ID)
     }
-    pub fn get_balance(account: &Account) -> i64 {
+    pub fn get_balance(account: &Account) -> u64 {
         account.tokens
     }
     pub fn process_transaction(
@@ -84,6 +85,10 @@ impl SystemProgram {
                         info!("Invalid account[1]");
                         Err(Error::InvalidArgument)?;
                     }
+                    if tokens > accounts[0].tokens {
+                        info!("Insufficient tokens in account[0]");
+                        Err(Error::ResultWithNegativeTokens(pix as u8))?;
+                    }
                     accounts[0].tokens -= tokens;
                     accounts[1].tokens += tokens;
                     accounts[1].program_id = program_id;
@@ -99,6 +104,10 @@ impl SystemProgram {
                 }
                 SystemProgram::Move { tokens } => {
                     //bank should be verifying correctness
+                    if tokens > accounts[0].tokens {
+                        info!("Insufficient tokens in account[0]");
+                        Err(Error::ResultWithNegativeTokens(pix as u8))?;
+                    }
                     accounts[0].tokens -= tokens;
                     accounts[1].tokens += tokens;
                 }
diff --git a/src/system_transaction.rs b/src/system_transaction.rs
index bab6bd42cc..9d7338f1aa 100644
--- a/src/system_transaction.rs
+++ b/src/system_transaction.rs
@@ -12,32 +12,32 @@ pub trait SystemTransaction {
         from_keypair: &Keypair,
         to: Pubkey,
         last_id: Hash,
-        tokens: i64,
+        tokens: u64,
         space: u64,
         program_id: Pubkey,
-        fee: i64,
+        fee: u64,
     ) -> Self;
 
-    fn system_assign(from_keypair: &Keypair, last_id: Hash, program_id: Pubkey, fee: i64) -> Self;
+    fn system_assign(from_keypair: &Keypair, last_id: Hash, program_id: Pubkey, fee: u64) -> Self;
 
-    fn system_new(from_keypair: &Keypair, to: Pubkey, tokens: i64, last_id: Hash) -> Self;
+    fn system_new(from_keypair: &Keypair, to: Pubkey, tokens: u64, last_id: Hash) -> Self;
 
     fn system_move(
         from_keypair: &Keypair,
         to: Pubkey,
-        tokens: i64,
+        tokens: u64,
         last_id: Hash,
-        fee: i64,
+        fee: u64,
     ) -> Self;
 
     fn system_move_many(
         from_keypair: &Keypair,
-        moves: &[(Pubkey, i64)],
+        moves: &[(Pubkey, u64)],
         last_id: Hash,
-        fee: i64,
+        fee: u64,
     ) -> Self;
 
-    fn system_spawn(from_keypair: &Keypair, last_id: Hash, fee: i64) -> Self;
+    fn system_spawn(from_keypair: &Keypair, last_id: Hash, fee: u64) -> Self;
 }
 
 impl SystemTransaction for Transaction {
@@ -46,10 +46,10 @@ impl SystemTransaction for Transaction {
         from_keypair: &Keypair,
         to: Pubkey,
         last_id: Hash,
-        tokens: i64,
+        tokens: u64,
         space: u64,
         program_id: Pubkey,
-        fee: i64,
+        fee: u64,
     ) -> Self {
         let create = SystemProgram::CreateAccount {
             tokens, //TODO, the tokens to allocate might need to be higher then 0 in the future
@@ -67,7 +67,7 @@ impl SystemTransaction for Transaction {
         )
     }
     /// Create and sign new SystemProgram::Assign transaction
-    fn system_assign(from_keypair: &Keypair, last_id: Hash, program_id: Pubkey, fee: i64) -> Self {
+    fn system_assign(from_keypair: &Keypair, last_id: Hash, program_id: Pubkey, fee: u64) -> Self {
         let assign = SystemProgram::Assign { program_id };
         let userdata = serialize(&assign).unwrap();
         Transaction::new(
@@ -80,16 +80,16 @@ impl SystemTransaction for Transaction {
         )
     }
     /// Create and sign new SystemProgram::CreateAccount transaction with some defaults
-    fn system_new(from_keypair: &Keypair, to: Pubkey, tokens: i64, last_id: Hash) -> Self {
+    fn system_new(from_keypair: &Keypair, to: Pubkey, tokens: u64, last_id: Hash) -> Self {
         Transaction::system_create(from_keypair, to, last_id, tokens, 0, Pubkey::default(), 0)
     }
     /// Create and sign new SystemProgram::Move transaction
     fn system_move(
         from_keypair: &Keypair,
         to: Pubkey,
-        tokens: i64,
+        tokens: u64,
         last_id: Hash,
-        fee: i64,
+        fee: u64,
     ) -> Self {
         let move_tokens = SystemProgram::Move { tokens };
         let userdata = serialize(&move_tokens).unwrap();
@@ -103,7 +103,7 @@ impl SystemTransaction for Transaction {
         )
     }
     /// Create and sign new SystemProgram::Move transaction to many destinations
-    fn system_move_many(from: &Keypair, moves: &[(Pubkey, i64)], last_id: Hash, fee: i64) -> Self {
+    fn system_move_many(from: &Keypair, moves: &[(Pubkey, u64)], last_id: Hash, fee: u64) -> Self {
         let instructions: Vec<_> = moves
             .iter()
             .enumerate()
@@ -127,7 +127,7 @@ impl SystemTransaction for Transaction {
         )
     }
     /// Create and sign new SystemProgram::Spawn transaction
-    fn system_spawn(from_keypair: &Keypair, last_id: Hash, fee: i64) -> Self {
+    fn system_spawn(from_keypair: &Keypair, last_id: Hash, fee: u64) -> Self {
         let spawn = SystemProgram::Spawn;
         let userdata = serialize(&spawn).unwrap();
         Transaction::new(
diff --git a/src/thin_client.rs b/src/thin_client.rs
index 07c880a607..ef76f43285 100644
--- a/src/thin_client.rs
+++ b/src/thin_client.rs
@@ -154,7 +154,7 @@ impl ThinClient {
         node_keypair: &Keypair,
         vote_account_id: Pubkey,
         last_id: &Hash,
-        num_tokens: i64,
+        num_tokens: u64,
     ) -> io::Result<Signature> {
         let tx =
             Transaction::vote_account_new(&node_keypair, vote_account_id, *last_id, num_tokens);
@@ -175,7 +175,7 @@ impl ThinClient {
     /// Creates, signs, and processes a Transaction. Useful for writing unit-tests.
     pub fn transfer(
         &self,
-        n: i64,
+        n: u64,
         keypair: &Keypair,
         to: Pubkey,
         last_id: &Hash,
@@ -215,7 +215,7 @@ impl ThinClient {
     /// Request the balance of the user holding `pubkey`. This method blocks
     /// until the server sends a response. If the response packet is dropped
     /// by the network, this method will hang indefinitely.
-    pub fn get_balance(&mut self, pubkey: &Pubkey) -> io::Result<i64> {
+    pub fn get_balance(&mut self, pubkey: &Pubkey) -> io::Result<u64> {
         trace!("get_balance sending request to {}", self.requests_addr);
         let req = Request::GetAccount { key: *pubkey };
         let data = serialize(&req).expect("serialize GetAccount in pub fn get_balance");
@@ -337,7 +337,7 @@ impl ThinClient {
         pubkey: &Pubkey,
         polling_frequency: &Duration,
         timeout: &Duration,
-    ) -> io::Result<i64> {
+    ) -> io::Result<u64> {
         let now = Instant::now();
         loop {
             match self.get_balance(&pubkey) {
@@ -356,7 +356,7 @@ impl ThinClient {
         }
     }
 
-    pub fn poll_get_balance(&mut self, pubkey: &Pubkey) -> io::Result<i64> {
+    pub fn poll_get_balance(&mut self, pubkey: &Pubkey) -> io::Result<u64> {
         self.poll_balance_with_timeout(pubkey, &Duration::from_millis(100), &Duration::from_secs(1))
     }
 
@@ -473,8 +473,8 @@ pub fn poll_gossip_for_leader(leader_ncp: SocketAddr, timeout: Option<u64>) -> R
 pub fn retry_get_balance(
     client: &mut ThinClient,
     bob_pubkey: &Pubkey,
-    expected_balance: Option<i64>,
-) -> Option<i64> {
+    expected_balance: Option<u64>,
+) -> Option<u64> {
     const LAST: usize = 30;
     for run in 0..LAST {
         let balance_result = client.poll_get_balance(bob_pubkey);
diff --git a/src/transaction.rs b/src/transaction.rs
index 1ac24e725f..af3e847602 100644
--- a/src/transaction.rs
+++ b/src/transaction.rs
@@ -40,7 +40,7 @@ pub struct Transaction {
     pub last_id: Hash,
 
     /// The number of tokens paid for processing and storage of this transaction.
-    pub fee: i64,
+    pub fee: u64,
 
     /// Keys identifying programs in the instructions vector.
     pub program_ids: Vec<Pubkey>,
@@ -56,7 +56,7 @@ impl Transaction {
         program_id: Pubkey,
         userdata: Vec<u8>,
         last_id: Hash,
-        fee: i64,
+        fee: u64,
     ) -> Self {
         let program_ids = vec![program_id];
         let instructions = vec![Instruction {
@@ -85,7 +85,7 @@ impl Transaction {
         from_keypair: &Keypair,
         keys: &[Pubkey],
         last_id: Hash,
-        fee: i64,
+        fee: u64,
         program_ids: Vec<Pubkey>,
         instructions: Vec<Instruction>,
     ) -> Self {
diff --git a/src/vote_transaction.rs b/src/vote_transaction.rs
index 00ae2f31be..07c8013427 100644
--- a/src/vote_transaction.rs
+++ b/src/vote_transaction.rs
@@ -15,24 +15,24 @@ use transaction::Transaction;
 use vote_program::{Vote, VoteInstruction, VoteProgram};
 
 pub trait VoteTransaction {
-    fn vote_new(vote_account: &Keypair, vote: Vote, last_id: Hash, fee: i64) -> Self;
+    fn vote_new(vote_account: &Keypair, vote: Vote, last_id: Hash, fee: u64) -> Self;
     fn vote_account_new(
         validator_id: &Keypair,
         new_vote_account_id: Pubkey,
         last_id: Hash,
-        num_tokens: i64,
+        num_tokens: u64,
     ) -> Self;
     fn vote_account_register(
         validator_id: &Keypair,
         vote_account_id: Pubkey,
         last_id: Hash,
-        fee: i64,
+        fee: u64,
     ) -> Self;
     fn get_votes(&self) -> Vec<(Pubkey, Vote, Hash)>;
 }
 
 impl VoteTransaction for Transaction {
-    fn vote_new(vote_account: &Keypair, vote: Vote, last_id: Hash, fee: i64) -> Self {
+    fn vote_new(vote_account: &Keypair, vote: Vote, last_id: Hash, fee: u64) -> Self {
         let instruction = VoteInstruction::NewVote(vote);
         let userdata = serialize(&instruction).expect("serialize instruction");
         Transaction::new(vote_account, &[], VoteProgram::id(), userdata, last_id, fee)
@@ -42,7 +42,7 @@ impl VoteTransaction for Transaction {
         validator_id: &Keypair,
         new_vote_account_id: Pubkey,
         last_id: Hash,
-        num_tokens: i64,
+        num_tokens: u64,
     ) -> Self {
         Transaction::system_create(
             validator_id,
@@ -59,7 +59,7 @@ impl VoteTransaction for Transaction {
         validator_id: &Keypair,
         vote_account_id: Pubkey,
         last_id: Hash,
-        fee: i64,
+        fee: u64,
     ) -> Self {
         let register_tx = VoteInstruction::RegisterAccount;
         let userdata = serialize(&register_tx).unwrap();
@@ -91,7 +91,7 @@ impl VoteTransaction for Transaction {
 pub fn create_vote_account(
     node_keypair: &Keypair,
     bank: &Bank,
-    num_tokens: i64,
+    num_tokens: u64,
     last_id: Hash,
 ) -> Result<Keypair> {
     let new_vote_account = Keypair::new();
diff --git a/src/wallet.rs b/src/wallet.rs
index 282c415287..93012898d4 100644
--- a/src/wallet.rs
+++ b/src/wallet.rs
@@ -37,7 +37,7 @@ const USERDATA_CHUNK_SIZE: usize = 256;
 #[derive(Debug, PartialEq)]
 pub enum WalletCommand {
     Address,
-    AirDrop(i64),
+    AirDrop(u64),
     Balance,
     Cancel(Pubkey),
     Confirm(Signature),
@@ -45,7 +45,7 @@ pub enum WalletCommand {
     GetTransactionCount,
     // Pay(tokens, to, timestamp, timestamp_pubkey, witness(es), cancelable)
     Pay(
-        i64,
+        u64,
         Pubkey,
         Option<DateTime<Utc>>,
         Option<Pubkey>,
@@ -308,7 +308,7 @@ pub fn process_command(config: &WalletConfig) -> Result<String, Box<error::Error
             let params = json!(format!("{}", config.id.pubkey()));
             let previous_balance = match RpcRequest::GetBalance
                 .make_rpc_request(&config.rpc_addr, 1, Some(params))?
-                .as_i64()
+                .as_u64()
             {
                 Some(tokens) => tokens,
                 None => Err(WalletError::RpcRequestError(
@@ -325,7 +325,7 @@ pub fn process_command(config: &WalletConfig) -> Result<String, Box<error::Error
                 let params = json!(format!("{}", config.id.pubkey()));
                 current_balance = RpcRequest::GetBalance
                     .make_rpc_request(&config.rpc_addr, 1, Some(params))?
-                    .as_i64()
+                    .as_u64()
                     .unwrap_or(previous_balance);
 
                 if previous_balance < current_balance {
@@ -344,7 +344,7 @@ pub fn process_command(config: &WalletConfig) -> Result<String, Box<error::Error
             let params = json!(format!("{}", config.id.pubkey()));
             let balance = RpcRequest::GetBalance
                 .make_rpc_request(&config.rpc_addr, 1, Some(params))?
-                .as_i64();
+                .as_u64();
             match balance {
                 Some(0) => Ok("No account found! Request an airdrop to get started.".to_string()),
                 Some(tokens) => Ok(format!("Your balance is: {:?}", tokens)),
@@ -385,7 +385,7 @@ pub fn process_command(config: &WalletConfig) -> Result<String, Box<error::Error
             let params = json!(format!("{}", config.id.pubkey()));
             let balance = RpcRequest::GetBalance
                 .make_rpc_request(&config.rpc_addr, 1, Some(params))?
-                .as_i64();
+                .as_u64();
             if let Some(tokens) = balance {
                 if tokens < 1 {
                     Err(WalletError::DynamicProgramError(
@@ -589,7 +589,7 @@ pub fn process_command(config: &WalletConfig) -> Result<String, Box<error::Error
             let params = json!(format!("{}", config.id.pubkey()));
             let balance = RpcRequest::GetBalance
                 .make_rpc_request(&config.rpc_addr, 1, Some(params))?
-                .as_i64();
+                .as_u64();
             if let Some(0) = balance {
                 request_airdrop(&config.drone_addr, &config.id.pubkey(), 1)?;
             }
@@ -608,7 +608,7 @@ pub fn process_command(config: &WalletConfig) -> Result<String, Box<error::Error
             let params = json!(format!("{}", config.id.pubkey()));
             let balance = RpcRequest::GetBalance
                 .make_rpc_request(&config.rpc_addr, 1, Some(params))?
-                .as_i64();
+                .as_u64();
             if let Some(0) = balance {
                 request_airdrop(&config.drone_addr, &config.id.pubkey(), 1)?;
             }
@@ -1314,21 +1314,21 @@ mod tests {
         let config_payer_balance = RpcRequest::GetBalance
             .make_rpc_request(&config_payer.rpc_addr, 1, Some(params))
             .unwrap()
-            .as_i64()
+            .as_u64()
             .unwrap();
         assert_eq!(config_payer_balance, 39);
         let params = json!(format!("{}", process_id));
         let contract_balance = RpcRequest::GetBalance
             .make_rpc_request(&config_payer.rpc_addr, 1, Some(params))
             .unwrap()
-            .as_i64()
+            .as_u64()
             .unwrap();
         assert_eq!(contract_balance, 11);
         let params = json!(format!("{}", bob_pubkey));
         let recipient_balance = RpcRequest::GetBalance
             .make_rpc_request(&config_payer.rpc_addr, 1, Some(params))
             .unwrap()
-            .as_i64()
+            .as_u64()
             .unwrap();
         assert_eq!(recipient_balance, 0);
 
@@ -1341,21 +1341,21 @@ mod tests {
         let config_payer_balance = RpcRequest::GetBalance
             .make_rpc_request(&config_payer.rpc_addr, 1, Some(params))
             .unwrap()
-            .as_i64()
+            .as_u64()
             .unwrap();
         assert_eq!(config_payer_balance, 39);
         let params = json!(format!("{}", process_id));
         let contract_balance = RpcRequest::GetBalance
             .make_rpc_request(&config_payer.rpc_addr, 1, Some(params))
             .unwrap()
-            .as_i64()
+            .as_u64()
             .unwrap();
         assert_eq!(contract_balance, 1);
         let params = json!(format!("{}", bob_pubkey));
         let recipient_balance = RpcRequest::GetBalance
             .make_rpc_request(&config_payer.rpc_addr, 1, Some(params))
             .unwrap()
-            .as_i64()
+            .as_u64()
             .unwrap();
         assert_eq!(recipient_balance, 10);
 
@@ -1437,21 +1437,21 @@ mod tests {
         let config_payer_balance = RpcRequest::GetBalance
             .make_rpc_request(&config_payer.rpc_addr, 1, Some(params))
             .unwrap()
-            .as_i64()
+            .as_u64()
             .unwrap();
         assert_eq!(config_payer_balance, 39);
         let params = json!(format!("{}", process_id));
         let contract_balance = RpcRequest::GetBalance
             .make_rpc_request(&config_payer.rpc_addr, 1, Some(params))
             .unwrap()
-            .as_i64()
+            .as_u64()
             .unwrap();
         assert_eq!(contract_balance, 11);
         let params = json!(format!("{}", bob_pubkey));
         let recipient_balance = RpcRequest::GetBalance
             .make_rpc_request(&config_payer.rpc_addr, 1, Some(params))
             .unwrap()
-            .as_i64()
+            .as_u64()
             .unwrap();
         assert_eq!(recipient_balance, 0);
 
@@ -1464,21 +1464,21 @@ mod tests {
         let config_payer_balance = RpcRequest::GetBalance
             .make_rpc_request(&config_payer.rpc_addr, 1, Some(params))
             .unwrap()
-            .as_i64()
+            .as_u64()
             .unwrap();
         assert_eq!(config_payer_balance, 39);
         let params = json!(format!("{}", process_id));
         let contract_balance = RpcRequest::GetBalance
             .make_rpc_request(&config_payer.rpc_addr, 1, Some(params))
             .unwrap()
-            .as_i64()
+            .as_u64()
             .unwrap();
         assert_eq!(contract_balance, 1);
         let params = json!(format!("{}", bob_pubkey));
         let recipient_balance = RpcRequest::GetBalance
             .make_rpc_request(&config_payer.rpc_addr, 1, Some(params))
             .unwrap()
-            .as_i64()
+            .as_u64()
             .unwrap();
         assert_eq!(recipient_balance, 10);
 
@@ -1562,21 +1562,21 @@ mod tests {
         let config_payer_balance = RpcRequest::GetBalance
             .make_rpc_request(&config_payer.rpc_addr, 1, Some(params))
             .unwrap()
-            .as_i64()
+            .as_u64()
             .unwrap();
         assert_eq!(config_payer_balance, 39);
         let params = json!(format!("{}", process_id));
         let contract_balance = RpcRequest::GetBalance
             .make_rpc_request(&config_payer.rpc_addr, 1, Some(params))
             .unwrap()
-            .as_i64()
+            .as_u64()
             .unwrap();
         assert_eq!(contract_balance, 11);
         let params = json!(format!("{}", bob_pubkey));
         let recipient_balance = RpcRequest::GetBalance
             .make_rpc_request(&config_payer.rpc_addr, 1, Some(params))
             .unwrap()
-            .as_i64()
+            .as_u64()
             .unwrap();
         assert_eq!(recipient_balance, 0);
 
@@ -1589,21 +1589,21 @@ mod tests {
         let config_payer_balance = RpcRequest::GetBalance
             .make_rpc_request(&config_payer.rpc_addr, 1, Some(params))
             .unwrap()
-            .as_i64()
+            .as_u64()
             .unwrap();
         assert_eq!(config_payer_balance, 49);
         let params = json!(format!("{}", process_id));
         let contract_balance = RpcRequest::GetBalance
             .make_rpc_request(&config_payer.rpc_addr, 1, Some(params))
             .unwrap()
-            .as_i64()
+            .as_u64()
             .unwrap();
         assert_eq!(contract_balance, 1);
         let params = json!(format!("{}", bob_pubkey));
         let recipient_balance = RpcRequest::GetBalance
             .make_rpc_request(&config_payer.rpc_addr, 1, Some(params))
             .unwrap()
-            .as_i64()
+            .as_u64()
             .unwrap();
         assert_eq!(recipient_balance, 0);
 
diff --git a/tests/multinode.rs b/tests/multinode.rs
index cc53a38ec5..f8a4b5d9fa 100644
--- a/tests/multinode.rs
+++ b/tests/multinode.rs
@@ -504,7 +504,7 @@ fn test_leader_restart_validator_start_from_old_ledger() -> result::Result<()> {
     let initial_leader_balance = 500;
     let (alice, ledger_path) = create_tmp_genesis(
         "leader_restart_validator_start_from_old_ledger",
-        100_000 + 500 * solana::window_service::MAX_REPAIR_BACKOFF as i64,
+        100_000 + 500 * solana::window_service::MAX_REPAIR_BACKOFF as u64,
         leader_keypair.pubkey(),
         initial_leader_balance,
     );
@@ -737,7 +737,7 @@ fn test_multi_node_dynamic_network() {
             info!("Took {} s to converge", duration_as_s(&start.elapsed()),);
             info!("Verifying signature of the last transaction in the validators");
 
-            let mut num_nodes_behind = 0i64;
+            let mut num_nodes_behind = 0u64;
             validators.retain(|server| {
                 let mut client = mk_client(&server.0);
                 trace!("{} checking signature", server.0.id);
@@ -855,11 +855,11 @@ fn test_leader_to_validator_transition() {
         // to ensure that each transaction is packaged as a single entry,
         // so that we can be sure leader rotation is triggered
         let result =
-            send_tx_and_retry_get_balance(&leader_info, &mint, &bob_pubkey, 1, Some(i as i64));
+            send_tx_and_retry_get_balance(&leader_info, &mint, &bob_pubkey, 1, Some(i as u64));
 
         // If the transaction wasn't reflected in the node, then we assume
         // the node has transitioned already
-        if result != Some(i as i64) {
+        if result != Some(i as u64) {
             break;
         }
 
@@ -989,7 +989,7 @@ fn test_leader_validator_basic() {
 
         // If the transaction wasn't reflected in the node, then we assume
         // the node has transitioned already
-        if result != Some(i as i64) {
+        if result != Some(i as u64) {
             break;
         }
 
@@ -1470,9 +1470,9 @@ fn send_tx_and_retry_get_balance(
     leader: &NodeInfo,
     alice: &Mint,
     bob_pubkey: &Pubkey,
-    transfer_amount: i64,
-    expected: Option<i64>,
-) -> Option<i64> {
+    transfer_amount: u64,
+    expected: Option<u64>,
+) -> Option<u64> {
     let mut client = mk_client(leader);
     trace!("getting leader last_id");
     let last_id = client.get_last_id();
@@ -1486,8 +1486,8 @@ fn retry_send_tx_and_retry_get_balance(
     leader: &NodeInfo,
     alice: &Mint,
     bob_pubkey: &Pubkey,
-    expected: Option<i64>,
-) -> Option<i64> {
+    expected: Option<u64>,
+) -> Option<u64> {
     let mut client = mk_client(leader);
     trace!("getting leader last_id");
     let last_id = client.get_last_id();