diff --git a/qa/pull-tester/rpc-tests.py b/qa/pull-tester/rpc-tests.py
index 7a30db68d..3d156a2e7 100755
--- a/qa/pull-tester/rpc-tests.py
+++ b/qa/pull-tester/rpc-tests.py
@@ -112,6 +112,7 @@ testScriptsExt = [
     'p2p-acceptblock.py',
     'mempool_packages.py',
     'maxuploadtarget.py',
+    'replace-by-fee.py',
 ]
 
 #Enable ZMQ tests
diff --git a/qa/replace-by-fee/.gitignore b/qa/replace-by-fee/.gitignore
new file mode 100644
index 000000000..b2c4f4657
--- /dev/null
+++ b/qa/replace-by-fee/.gitignore
@@ -0,0 +1 @@
+python-bitcoinlib
diff --git a/qa/replace-by-fee/README.md b/qa/replace-by-fee/README.md
new file mode 100644
index 000000000..baad86de9
--- /dev/null
+++ b/qa/replace-by-fee/README.md
@@ -0,0 +1,13 @@
+Replace-by-fee regression tests
+===============================
+
+First get version v0.5.0 of the python-bitcoinlib library. In this directory
+run:
+
+    git clone -n https://github.com/petertodd/python-bitcoinlib
+    (cd python-bitcoinlib && git checkout 8270bfd9c6ac37907d75db3d8b9152d61c7255cd)
+
+Then run the tests themselves with a bitcoind available running in regtest
+mode:
+
+    ./rbf-tests.py
diff --git a/qa/replace-by-fee/rbf-tests.py b/qa/replace-by-fee/rbf-tests.py
new file mode 100755
index 000000000..1ee6c8387
--- /dev/null
+++ b/qa/replace-by-fee/rbf-tests.py
@@ -0,0 +1,360 @@
+#!/usr/bin/env python3
+# Copyright (c) 2015 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#
+# Test replace-by-fee
+#
+
+import os
+import sys
+
+# Add python-bitcoinlib to module search path, prior to any system-wide
+# python-bitcoinlib.
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), "python-bitcoinlib"))
+
+import unittest
+
+import bitcoin
+bitcoin.SelectParams('regtest')
+
+import bitcoin.rpc
+
+from bitcoin.core import *
+from bitcoin.core.script import *
+from bitcoin.wallet import *
+
+MAX_REPLACEMENT_LIMIT = 100
+
+class Test_ReplaceByFee(unittest.TestCase):
+    proxy = None
+
+    @classmethod
+    def setUpClass(cls):
+        if cls.proxy is None:
+            cls.proxy = bitcoin.rpc.Proxy()
+
+    @classmethod
+    def mine_mempool(cls):
+        """Mine until mempool is empty"""
+        mempool_size = 1
+        while mempool_size:
+            cls.proxy.call('generate', 1)
+            new_mempool_size = len(cls.proxy.getrawmempool())
+
+            # It's possible to get stuck in a loop here if the mempool has
+            # transactions that can't be mined.
+            assert(new_mempool_size != mempool_size)
+            mempool_size = new_mempool_size
+
+    @classmethod
+    def tearDownClass(cls):
+        # Make sure mining works
+        cls.mine_mempool()
+
+    def make_txout(self, amount, confirmed=True, scriptPubKey=CScript([1])):
+        """Create a txout with a given amount and scriptPubKey
+
+        Mines coins as needed.
+
+        confirmed - txouts created will be confirmed in the blockchain;
+                    unconfirmed otherwise.
+        """
+        fee = 1*COIN
+        while self.proxy.getbalance() < amount + fee:
+            self.proxy.call('generate', 100)
+
+        addr = P2SHBitcoinAddress.from_redeemScript(CScript([]))
+        txid = self.proxy.sendtoaddress(addr, amount + fee)
+
+        tx1 = self.proxy.getrawtransaction(txid)
+
+        i = None
+        for i, txout in enumerate(tx1.vout):
+            if txout.scriptPubKey == addr.to_scriptPubKey():
+                break
+        assert i is not None
+
+        tx2 = CTransaction([CTxIn(COutPoint(txid, i), CScript([1, CScript([])]), nSequence=0)],
+                           [CTxOut(amount, scriptPubKey)])
+
+        tx2_txid = self.proxy.sendrawtransaction(tx2, True)
+
+        # If requested, ensure txouts are confirmed.
+        if confirmed:
+            self.mine_mempool()
+
+        return COutPoint(tx2_txid, 0)
+
+    def test_simple_doublespend(self):
+        """Simple doublespend"""
+        tx0_outpoint = self.make_txout(1.1*COIN)
+
+        tx1a = CTransaction([CTxIn(tx0_outpoint, nSequence=0)],
+                            [CTxOut(1*COIN, CScript([b'a']))])
+        tx1a_txid = self.proxy.sendrawtransaction(tx1a, True)
+
+        # Should fail because we haven't changed the fee
+        tx1b = CTransaction([CTxIn(tx0_outpoint, nSequence=0)],
+                            [CTxOut(1*COIN, CScript([b'b']))])
+
+        try:
+            tx1b_txid = self.proxy.sendrawtransaction(tx1b, True)
+        except bitcoin.rpc.JSONRPCException as exp:
+            self.assertEqual(exp.error['code'], -26) # insufficient fee
+        else:
+            self.fail()
+
+        # Extra 0.1 BTC fee
+        tx1b = CTransaction([CTxIn(tx0_outpoint, nSequence=0)],
+                            [CTxOut(0.9*COIN, CScript([b'b']))])
+        tx1b_txid = self.proxy.sendrawtransaction(tx1b, True)
+
+        # tx1a is in fact replaced
+        with self.assertRaises(IndexError):
+            self.proxy.getrawtransaction(tx1a_txid)
+
+        self.assertEqual(tx1b, self.proxy.getrawtransaction(tx1b_txid))
+
+    def test_doublespend_chain(self):
+        """Doublespend of a long chain"""
+
+        initial_nValue = 50*COIN
+        tx0_outpoint = self.make_txout(initial_nValue)
+
+        prevout = tx0_outpoint
+        remaining_value = initial_nValue
+        chain_txids = []
+        while remaining_value > 10*COIN:
+            remaining_value -= 1*COIN
+            tx = CTransaction([CTxIn(prevout, nSequence=0)],
+                              [CTxOut(remaining_value, CScript([1]))])
+            txid = self.proxy.sendrawtransaction(tx, True)
+            chain_txids.append(txid)
+            prevout = COutPoint(txid, 0)
+
+        # Whether the double-spend is allowed is evaluated by including all
+        # child fees - 40 BTC - so this attempt is rejected.
+        dbl_tx = CTransaction([CTxIn(tx0_outpoint, nSequence=0)],
+                              [CTxOut(initial_nValue - 30*COIN, CScript([1]))])
+
+        try:
+            self.proxy.sendrawtransaction(dbl_tx, True)
+        except bitcoin.rpc.JSONRPCException as exp:
+            self.assertEqual(exp.error['code'], -26) # insufficient fee
+        else:
+            self.fail()
+
+        # Accepted with sufficient fee
+        dbl_tx = CTransaction([CTxIn(tx0_outpoint, nSequence=0)],
+                              [CTxOut(1*COIN, CScript([1]))])
+        self.proxy.sendrawtransaction(dbl_tx, True)
+
+        for doublespent_txid in chain_txids:
+            with self.assertRaises(IndexError):
+                self.proxy.getrawtransaction(doublespent_txid)
+
+    def test_doublespend_tree(self):
+        """Doublespend of a big tree of transactions"""
+
+        initial_nValue = 50*COIN
+        tx0_outpoint = self.make_txout(initial_nValue)
+
+        def branch(prevout, initial_value, max_txs, *, tree_width=5, fee=0.0001*COIN, _total_txs=None):
+            if _total_txs is None:
+                _total_txs = [0]
+            if _total_txs[0] >= max_txs:
+                return
+
+            txout_value = (initial_value - fee) // tree_width
+            if txout_value < fee:
+                return
+
+            vout = [CTxOut(txout_value, CScript([i+1]))
+                    for i in range(tree_width)]
+            tx = CTransaction([CTxIn(prevout, nSequence=0)],
+                              vout)
+
+            self.assertTrue(len(tx.serialize()) < 100000)
+            txid = self.proxy.sendrawtransaction(tx, True)
+            yield tx
+            _total_txs[0] += 1
+
+            for i, txout in enumerate(tx.vout):
+                yield from branch(COutPoint(txid, i), txout_value,
+                                  max_txs,
+                                  tree_width=tree_width, fee=fee,
+                                  _total_txs=_total_txs)
+
+        fee = 0.0001*COIN
+        n = MAX_REPLACEMENT_LIMIT
+        tree_txs = list(branch(tx0_outpoint, initial_nValue, n, fee=fee))
+        self.assertEqual(len(tree_txs), n)
+
+        # Attempt double-spend, will fail because too little fee paid
+        dbl_tx = CTransaction([CTxIn(tx0_outpoint, nSequence=0)],
+                              [CTxOut(initial_nValue - fee*n, CScript([1]))])
+        try:
+            self.proxy.sendrawtransaction(dbl_tx, True)
+        except bitcoin.rpc.JSONRPCException as exp:
+            self.assertEqual(exp.error['code'], -26) # insufficient fee
+        else:
+            self.fail()
+
+        # 1 BTC fee is enough
+        dbl_tx = CTransaction([CTxIn(tx0_outpoint, nSequence=0)],
+                              [CTxOut(initial_nValue - fee*n - 1*COIN, CScript([1]))])
+        self.proxy.sendrawtransaction(dbl_tx, True)
+
+        for tx in tree_txs:
+            with self.assertRaises(IndexError):
+                self.proxy.getrawtransaction(tx.GetHash())
+
+        # Try again, but with more total transactions than the "max txs
+        # double-spent at once" anti-DoS limit.
+        for n in (MAX_REPLACEMENT_LIMIT, MAX_REPLACEMENT_LIMIT*2):
+            fee = 0.0001*COIN
+            tx0_outpoint = self.make_txout(initial_nValue)
+            tree_txs = list(branch(tx0_outpoint, initial_nValue, n, fee=fee))
+            self.assertEqual(len(tree_txs), n)
+
+            dbl_tx = CTransaction([CTxIn(tx0_outpoint, nSequence=0)],
+                                  [CTxOut(initial_nValue - fee*n, CScript([1]))])
+            try:
+                self.proxy.sendrawtransaction(dbl_tx, True)
+            except bitcoin.rpc.JSONRPCException as exp:
+                self.assertEqual(exp.error['code'], -26)
+            else:
+                self.fail()
+
+            for tx in tree_txs:
+                self.proxy.getrawtransaction(tx.GetHash())
+
+    def test_replacement_feeperkb(self):
+        """Replacement requires fee-per-KB to be higher"""
+        tx0_outpoint = self.make_txout(1.1*COIN)
+
+        tx1a = CTransaction([CTxIn(tx0_outpoint, nSequence=0)],
+                            [CTxOut(1*COIN, CScript([b'a']))])
+        tx1a_txid = self.proxy.sendrawtransaction(tx1a, True)
+
+        # Higher fee, but the fee per KB is much lower, so the replacement is
+        # rejected.
+        tx1b = CTransaction([CTxIn(tx0_outpoint, nSequence=0)],
+                            [CTxOut(0.001*COIN,
+                                    CScript([b'a'*999000]))])
+
+        try:
+            tx1b_txid = self.proxy.sendrawtransaction(tx1b, True)
+        except bitcoin.rpc.JSONRPCException as exp:
+            self.assertEqual(exp.error['code'], -26) # insufficient fee
+        else:
+            self.fail()
+
+    def test_spends_of_conflicting_outputs(self):
+        """Replacements that spend conflicting tx outputs are rejected"""
+        utxo1 = self.make_txout(1.2*COIN)
+        utxo2 = self.make_txout(3.0*COIN)
+
+        tx1a = CTransaction([CTxIn(utxo1, nSequence=0)],
+                            [CTxOut(1.1*COIN, CScript([b'a']))])
+        tx1a_txid = self.proxy.sendrawtransaction(tx1a, True)
+
+        # Direct spend an output of the transaction we're replacing.
+        tx2 = CTransaction([CTxIn(utxo1, nSequence=0), CTxIn(utxo2, nSequence=0),
+                            CTxIn(COutPoint(tx1a_txid, 0), nSequence=0)],
+                           tx1a.vout)
+
+        try:
+            tx2_txid = self.proxy.sendrawtransaction(tx2, True)
+        except bitcoin.rpc.JSONRPCException as exp:
+            self.assertEqual(exp.error['code'], -26)
+        else:
+            self.fail()
+
+        # Spend tx1a's output to test the indirect case.
+        tx1b = CTransaction([CTxIn(COutPoint(tx1a_txid, 0), nSequence=0)],
+                            [CTxOut(1.0*COIN, CScript([b'a']))])
+        tx1b_txid = self.proxy.sendrawtransaction(tx1b, True)
+
+        tx2 = CTransaction([CTxIn(utxo1, nSequence=0), CTxIn(utxo2, nSequence=0),
+                            CTxIn(COutPoint(tx1b_txid, 0))],
+                           tx1a.vout)
+
+        try:
+            tx2_txid = self.proxy.sendrawtransaction(tx2, True)
+        except bitcoin.rpc.JSONRPCException as exp:
+            self.assertEqual(exp.error['code'], -26)
+        else:
+            self.fail()
+
+    def test_new_unconfirmed_inputs(self):
+        """Replacements that add new unconfirmed inputs are rejected"""
+        confirmed_utxo = self.make_txout(1.1*COIN)
+        unconfirmed_utxo = self.make_txout(0.1*COIN, False)
+
+        tx1 = CTransaction([CTxIn(confirmed_utxo)],
+                           [CTxOut(1.0*COIN, CScript([b'a']))])
+        tx1_txid = self.proxy.sendrawtransaction(tx1, True)
+
+        tx2 = CTransaction([CTxIn(confirmed_utxo), CTxIn(unconfirmed_utxo)],
+                           tx1.vout)
+
+        try:
+            tx2_txid = self.proxy.sendrawtransaction(tx2, True)
+        except bitcoin.rpc.JSONRPCException as exp:
+            self.assertEqual(exp.error['code'], -26)
+        else:
+            self.fail()
+
+    def test_too_many_replacements(self):
+        """Replacements that evict too many transactions are rejected"""
+        # Try directly replacing more than MAX_REPLACEMENT_LIMIT
+        # transactions
+
+        # Start by creating a single transaction with many outputs
+        initial_nValue = 10*COIN
+        utxo = self.make_txout(initial_nValue)
+        fee = 0.0001*COIN
+        split_value = int((initial_nValue-fee)/(MAX_REPLACEMENT_LIMIT+1))
+        actual_fee = initial_nValue - split_value*(MAX_REPLACEMENT_LIMIT+1)
+
+        outputs = []
+        for i in range(MAX_REPLACEMENT_LIMIT+1):
+            outputs.append(CTxOut(split_value, CScript([1])))
+
+        splitting_tx = CTransaction([CTxIn(utxo, nSequence=0)], outputs)
+        txid = self.proxy.sendrawtransaction(splitting_tx, True)
+
+        # Now spend each of those outputs individually
+        for i in range(MAX_REPLACEMENT_LIMIT+1):
+            tx_i = CTransaction([CTxIn(COutPoint(txid, i), nSequence=0)],
+                                [CTxOut(split_value-fee, CScript([b'a']))])
+            self.proxy.sendrawtransaction(tx_i, True)
+
+        # Now create doublespend of the whole lot, should fail
+        # Need a big enough fee to cover all spending transactions and have
+        # a higher fee rate
+        double_spend_value = (split_value-100*fee)*(MAX_REPLACEMENT_LIMIT+1)
+        inputs = []
+        for i in range(MAX_REPLACEMENT_LIMIT+1):
+            inputs.append(CTxIn(COutPoint(txid, i), nSequence=0))
+        double_tx = CTransaction(inputs, [CTxOut(double_spend_value, CScript([b'a']))])
+
+        try:
+            self.proxy.sendrawtransaction(double_tx, True)
+        except bitcoin.rpc.JSONRPCException as exp:
+            self.assertEqual(exp.error['code'], -26)
+            self.assertEqual("too many potential replacements" in exp.error['message'], True)
+        else:
+            self.fail()
+
+        # If we remove an input, it should pass
+        double_tx = CTransaction(inputs[0:-1],
+                                 [CTxOut(double_spend_value, CScript([b'a']))])
+
+        self.proxy.sendrawtransaction(double_tx, True)
+
+if __name__ == '__main__':
+    unittest.main()
diff --git a/qa/rpc-tests/replace-by-fee.py b/qa/rpc-tests/replace-by-fee.py
new file mode 100755
index 000000000..537a1ed8d
--- /dev/null
+++ b/qa/rpc-tests/replace-by-fee.py
@@ -0,0 +1,512 @@
+#!/usr/bin/env python2
+# Copyright (c) 2014-2015 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#
+# Test replace by fee code
+#
+
+from test_framework.test_framework import BitcoinTestFramework
+from test_framework.util import *
+from test_framework.script import *
+from test_framework.mininode import *
+import binascii
+
+COIN = 100000000
+MAX_REPLACEMENT_LIMIT = 100
+
+def satoshi_round(amount):
+    return Decimal(amount).quantize(Decimal('0.00000001'), rounding=ROUND_DOWN)
+
+def txToHex(tx):
+    return binascii.hexlify(tx.serialize()).decode('utf-8')
+
+def make_utxo(node, amount, confirmed=True, scriptPubKey=CScript([1])):
+    """Create a txout with a given amount and scriptPubKey
+
+    Mines coins as needed.
+
+    confirmed - txouts created will be confirmed in the blockchain;
+                unconfirmed otherwise.
+    """
+    fee = 1*COIN
+    while node.getbalance() < satoshi_round((amount + fee)/COIN):
+        node.generate(100)
+        #print (node.getbalance(), amount, fee)
+
+    new_addr = node.getnewaddress()
+    #print new_addr
+    txid = node.sendtoaddress(new_addr, satoshi_round((amount+fee)/COIN))
+    tx1 = node.getrawtransaction(txid, 1)
+    txid = int(txid, 16)
+    i = None
+
+    for i, txout in enumerate(tx1['vout']):
+        #print i, txout['scriptPubKey']['addresses']
+        if txout['scriptPubKey']['addresses'] == [new_addr]:
+            #print i
+            break
+    assert i is not None
+
+    tx2 = CTransaction()
+    tx2.vin = [CTxIn(COutPoint(txid, i))]
+    tx2.vout = [CTxOut(amount, scriptPubKey)]
+    tx2.rehash()
+
+    tx2_hex = binascii.hexlify(tx2.serialize()).decode('utf-8')
+    #print tx2_hex
+
+    signed_tx = node.signrawtransaction(binascii.hexlify(tx2.serialize()).decode('utf-8'))
+
+    txid = node.sendrawtransaction(signed_tx['hex'], True)
+
+    # If requested, ensure txouts are confirmed.
+    if confirmed:
+        while len(node.getrawmempool()):
+            node.generate(1)
+
+    return COutPoint(int(txid, 16), 0)
+
+class ReplaceByFeeTest(BitcoinTestFramework):
+
+    def setup_network(self):
+        self.nodes = []
+        self.nodes.append(start_node(0, self.options.tmpdir, ["-maxorphantx=1000",
+                                                              "-relaypriority=0", "-whitelist=127.0.0.1"]))
+        self.is_network_split = False
+
+    def run_test(self):
+        make_utxo(self.nodes[0], 1*COIN)
+
+        print "Running test simple doublespend..."
+        self.test_simple_doublespend()
+
+        print "Running test doublespend chain..."
+        self.test_doublespend_chain()
+
+        print "Running test doublespend tree..."
+        self.test_doublespend_tree()
+
+        print "Running test replacement feeperkb..."
+        self.test_replacement_feeperkb()
+
+        print "Running test spends of conflicting outputs..."
+        self.test_spends_of_conflicting_outputs()
+
+        print "Running test new unconfirmed inputs..."
+        self.test_new_unconfirmed_inputs()
+
+        print "Running test too many replacements..."
+        self.test_too_many_replacements()
+
+        print "Running test opt-in..."
+        self.test_opt_in()
+
+        print "Passed\n"
+
+    def test_simple_doublespend(self):
+        """Simple doublespend"""
+        tx0_outpoint = make_utxo(self.nodes[0], 1.1*COIN)
+
+        tx1a = CTransaction()
+        tx1a.vin = [CTxIn(tx0_outpoint, nSequence=0)]
+        tx1a.vout = [CTxOut(1*COIN, CScript([b'a']))]
+        tx1a_hex = txToHex(tx1a)
+        tx1a_txid = self.nodes[0].sendrawtransaction(tx1a_hex, True)
+
+        # Should fail because we haven't changed the fee
+        tx1b = CTransaction()
+        tx1b.vin = [CTxIn(tx0_outpoint, nSequence=0)]
+        tx1b.vout = [CTxOut(1*COIN, CScript([b'b']))]
+        tx1b_hex = txToHex(tx1b)
+
+        try:
+            tx1b_txid = self.nodes[0].sendrawtransaction(tx1b_hex, True)
+        except JSONRPCException as exp:
+            assert_equal(exp.error['code'], -26) # insufficient fee
+        else:
+            assert(False)
+
+        # Extra 0.1 BTC fee
+        tx1b = CTransaction()
+        tx1b.vin = [CTxIn(tx0_outpoint, nSequence=0)]
+        tx1b.vout = [CTxOut(0.9*COIN, CScript([b'b']))]
+        tx1b_hex = txToHex(tx1b)
+        tx1b_txid = self.nodes[0].sendrawtransaction(tx1b_hex, True)
+
+        mempool = self.nodes[0].getrawmempool()
+
+        assert (tx1a_txid not in mempool)
+        assert (tx1b_txid in mempool)
+
+        assert_equal(tx1b_hex, self.nodes[0].getrawtransaction(tx1b_txid))
+
+    def test_doublespend_chain(self):
+        """Doublespend of a long chain"""
+
+        initial_nValue = 50*COIN
+        tx0_outpoint = make_utxo(self.nodes[0], initial_nValue)
+
+        prevout = tx0_outpoint
+        remaining_value = initial_nValue
+        chain_txids = []
+        while remaining_value > 10*COIN:
+            remaining_value -= 1*COIN
+            tx = CTransaction()
+            tx.vin = [CTxIn(prevout, nSequence=0)]
+            tx.vout = [CTxOut(remaining_value, CScript([1]))]
+            tx_hex = txToHex(tx)
+            txid = self.nodes[0].sendrawtransaction(tx_hex, True)
+            chain_txids.append(txid)
+            prevout = COutPoint(int(txid, 16), 0)
+
+        # Whether the double-spend is allowed is evaluated by including all
+        # child fees - 40 BTC - so this attempt is rejected.
+        dbl_tx = CTransaction()
+        dbl_tx.vin = [CTxIn(tx0_outpoint, nSequence=0)]
+        dbl_tx.vout = [CTxOut(initial_nValue - 30*COIN, CScript([1]))]
+        dbl_tx_hex = txToHex(dbl_tx)
+
+        try:
+            self.nodes[0].sendrawtransaction(dbl_tx_hex, True)
+        except JSONRPCException as exp:
+            assert_equal(exp.error['code'], -26) # insufficient fee
+        else:
+            assert(False) # transaction mistakenly accepted!
+
+        # Accepted with sufficient fee
+        dbl_tx = CTransaction()
+        dbl_tx.vin = [CTxIn(tx0_outpoint, nSequence=0)]
+        dbl_tx.vout = [CTxOut(1*COIN, CScript([1]))]
+        dbl_tx_hex = txToHex(dbl_tx)
+        self.nodes[0].sendrawtransaction(dbl_tx_hex, True)
+
+        mempool = self.nodes[0].getrawmempool()
+        for doublespent_txid in chain_txids:
+            assert(doublespent_txid not in mempool)
+
+    def test_doublespend_tree(self):
+        """Doublespend of a big tree of transactions"""
+
+        initial_nValue = 50*COIN
+        tx0_outpoint = make_utxo(self.nodes[0], initial_nValue)
+
+        def branch(prevout, initial_value, max_txs, tree_width=5, fee=0.0001*COIN, _total_txs=None):
+            if _total_txs is None:
+                _total_txs = [0]
+            if _total_txs[0] >= max_txs:
+                return
+
+            txout_value = (initial_value - fee) // tree_width
+            if txout_value < fee:
+                return
+
+            vout = [CTxOut(txout_value, CScript([i+1]))
+                    for i in range(tree_width)]
+            tx = CTransaction()
+            tx.vin = [CTxIn(prevout, nSequence=0)]
+            tx.vout = vout
+            tx_hex = txToHex(tx)
+
+            assert(len(tx.serialize()) < 100000)
+            txid = self.nodes[0].sendrawtransaction(tx_hex, True)
+            yield tx
+            _total_txs[0] += 1
+
+            txid = int(txid, 16)
+
+            for i, txout in enumerate(tx.vout):
+                for x in branch(COutPoint(txid, i), txout_value,
+                                  max_txs,
+                                  tree_width=tree_width, fee=fee,
+                                  _total_txs=_total_txs):
+                    yield x
+
+        fee = 0.0001*COIN
+        n = MAX_REPLACEMENT_LIMIT
+        tree_txs = list(branch(tx0_outpoint, initial_nValue, n, fee=fee))
+        assert_equal(len(tree_txs), n)
+
+        # Attempt double-spend, will fail because too little fee paid
+        dbl_tx = CTransaction()
+        dbl_tx.vin = [CTxIn(tx0_outpoint, nSequence=0)]
+        dbl_tx.vout = [CTxOut(initial_nValue - fee*n, CScript([1]))]
+        dbl_tx_hex = txToHex(dbl_tx)
+        try:
+            self.nodes[0].sendrawtransaction(dbl_tx_hex, True)
+        except JSONRPCException as exp:
+            assert_equal(exp.error['code'], -26) # insufficient fee
+        else:
+            assert(False)
+
+        # 1 BTC fee is enough
+        dbl_tx = CTransaction()
+        dbl_tx.vin = [CTxIn(tx0_outpoint, nSequence=0)]
+        dbl_tx.vout = [CTxOut(initial_nValue - fee*n - 1*COIN, CScript([1]))]
+        dbl_tx_hex = txToHex(dbl_tx)
+        self.nodes[0].sendrawtransaction(dbl_tx_hex, True)
+
+        mempool = self.nodes[0].getrawmempool()
+
+        for tx in tree_txs:
+            tx.rehash()
+            assert (tx.hash not in mempool)
+
+        # Try again, but with more total transactions than the "max txs
+        # double-spent at once" anti-DoS limit.
+        for n in (MAX_REPLACEMENT_LIMIT+1, MAX_REPLACEMENT_LIMIT*2):
+            fee = 0.0001*COIN
+            tx0_outpoint = make_utxo(self.nodes[0], initial_nValue)
+            tree_txs = list(branch(tx0_outpoint, initial_nValue, n, fee=fee))
+            assert_equal(len(tree_txs), n)
+
+            dbl_tx = CTransaction()
+            dbl_tx.vin = [CTxIn(tx0_outpoint, nSequence=0)]
+            dbl_tx.vout = [CTxOut(initial_nValue - 2*fee*n, CScript([1]))]
+            dbl_tx_hex = txToHex(dbl_tx)
+            try:
+                self.nodes[0].sendrawtransaction(dbl_tx_hex, True)
+            except JSONRPCException as exp:
+                assert_equal(exp.error['code'], -26)
+                assert_equal("too many potential replacements" in exp.error['message'], True)
+            else:
+                assert(False)
+
+            for tx in tree_txs:
+                tx.rehash()
+                self.nodes[0].getrawtransaction(tx.hash)
+
+    def test_replacement_feeperkb(self):
+        """Replacement requires fee-per-KB to be higher"""
+        tx0_outpoint = make_utxo(self.nodes[0], 1.1*COIN)
+
+        tx1a = CTransaction()
+        tx1a.vin = [CTxIn(tx0_outpoint, nSequence=0)]
+        tx1a.vout = [CTxOut(1*COIN, CScript([b'a']))]
+        tx1a_hex = txToHex(tx1a)
+        tx1a_txid = self.nodes[0].sendrawtransaction(tx1a_hex, True)
+
+        # Higher fee, but the fee per KB is much lower, so the replacement is
+        # rejected.
+        tx1b = CTransaction()
+        tx1b.vin = [CTxIn(tx0_outpoint, nSequence=0)]
+        tx1b.vout = [CTxOut(0.001*COIN, CScript([b'a'*999000]))]
+        tx1b_hex = txToHex(tx1b)
+
+        try:
+            tx1b_txid = self.nodes[0].sendrawtransaction(tx1b_hex, True)
+        except JSONRPCException as exp:
+            assert_equal(exp.error['code'], -26) # insufficient fee
+        else:
+            assert(False)
+
+    def test_spends_of_conflicting_outputs(self):
+        """Replacements that spend conflicting tx outputs are rejected"""
+        utxo1 = make_utxo(self.nodes[0], 1.2*COIN)
+        utxo2 = make_utxo(self.nodes[0], 3.0*COIN)
+
+        tx1a = CTransaction()
+        tx1a.vin = [CTxIn(utxo1, nSequence=0)]
+        tx1a.vout = [CTxOut(1.1*COIN, CScript([b'a']))]
+        tx1a_hex = txToHex(tx1a)
+        tx1a_txid = self.nodes[0].sendrawtransaction(tx1a_hex, True)
+
+        tx1a_txid = int(tx1a_txid, 16)
+
+        # Direct spend an output of the transaction we're replacing.
+        tx2 = CTransaction()
+        tx2.vin = [CTxIn(utxo1, nSequence=0), CTxIn(utxo2, nSequence=0)]
+        tx2.vin.append(CTxIn(COutPoint(tx1a_txid, 0), nSequence=0))
+        tx2.vout = tx1a.vout
+        tx2_hex = txToHex(tx2)
+
+        try:
+            tx2_txid = self.nodes[0].sendrawtransaction(tx2_hex, True)
+        except JSONRPCException as exp:
+            assert_equal(exp.error['code'], -26)
+        else:
+            assert(False)
+
+        # Spend tx1a's output to test the indirect case.
+        tx1b = CTransaction()
+        tx1b.vin = [CTxIn(COutPoint(tx1a_txid, 0), nSequence=0)]
+        tx1b.vout = [CTxOut(1.0*COIN, CScript([b'a']))]
+        tx1b_hex = txToHex(tx1b)
+        tx1b_txid = self.nodes[0].sendrawtransaction(tx1b_hex, True)
+        tx1b_txid = int(tx1b_txid, 16)
+
+        tx2 = CTransaction()
+        tx2.vin = [CTxIn(utxo1, nSequence=0), CTxIn(utxo2, nSequence=0),
+                   CTxIn(COutPoint(tx1b_txid, 0))]
+        tx2.vout = tx1a.vout
+        tx2_hex = txToHex(tx2)
+
+        try:
+            tx2_txid = self.nodes[0].sendrawtransaction(tx2_hex, True)
+        except JSONRPCException as exp:
+            assert_equal(exp.error['code'], -26)
+        else:
+            assert(False)
+
+    def test_new_unconfirmed_inputs(self):
+        """Replacements that add new unconfirmed inputs are rejected"""
+        confirmed_utxo = make_utxo(self.nodes[0], 1.1*COIN)
+        unconfirmed_utxo = make_utxo(self.nodes[0], 0.1*COIN, False)
+
+        tx1 = CTransaction()
+        tx1.vin = [CTxIn(confirmed_utxo)]
+        tx1.vout = [CTxOut(1.0*COIN, CScript([b'a']))]
+        tx1_hex = txToHex(tx1)
+        tx1_txid = self.nodes[0].sendrawtransaction(tx1_hex, True)
+
+        tx2 = CTransaction()
+        tx2.vin = [CTxIn(confirmed_utxo), CTxIn(unconfirmed_utxo)]
+        tx2.vout = tx1.vout
+        tx2_hex = txToHex(tx2)
+
+        try:
+            tx2_txid = self.nodes[0].sendrawtransaction(tx2_hex, True)
+        except JSONRPCException as exp:
+            assert_equal(exp.error['code'], -26)
+        else:
+            assert(False)
+
+    def test_too_many_replacements(self):
+        """Replacements that evict too many transactions are rejected"""
+        # Try directly replacing more than MAX_REPLACEMENT_LIMIT
+        # transactions
+
+        # Start by creating a single transaction with many outputs
+        initial_nValue = 10*COIN
+        utxo = make_utxo(self.nodes[0], initial_nValue)
+        fee = 0.0001*COIN
+        split_value = int((initial_nValue-fee)/(MAX_REPLACEMENT_LIMIT+1))
+        actual_fee = initial_nValue - split_value*(MAX_REPLACEMENT_LIMIT+1)
+
+        outputs = []
+        for i in range(MAX_REPLACEMENT_LIMIT+1):
+            outputs.append(CTxOut(split_value, CScript([1])))
+
+        splitting_tx = CTransaction()
+        splitting_tx.vin = [CTxIn(utxo, nSequence=0)]
+        splitting_tx.vout = outputs
+        splitting_tx_hex = txToHex(splitting_tx)
+
+        txid = self.nodes[0].sendrawtransaction(splitting_tx_hex, True)
+        txid = int(txid, 16)
+
+        # Now spend each of those outputs individually
+        for i in range(MAX_REPLACEMENT_LIMIT+1):
+            tx_i = CTransaction()
+            tx_i.vin = [CTxIn(COutPoint(txid, i), nSequence=0)]
+            tx_i.vout = [CTxOut(split_value-fee, CScript([b'a']))]
+            tx_i_hex = txToHex(tx_i)
+            self.nodes[0].sendrawtransaction(tx_i_hex, True)
+
+        # Now create doublespend of the whole lot; should fail.
+        # Need a big enough fee to cover all spending transactions and have
+        # a higher fee rate
+        double_spend_value = (split_value-100*fee)*(MAX_REPLACEMENT_LIMIT+1)
+        inputs = []
+        for i in range(MAX_REPLACEMENT_LIMIT+1):
+            inputs.append(CTxIn(COutPoint(txid, i), nSequence=0))
+        double_tx = CTransaction()
+        double_tx.vin = inputs
+        double_tx.vout = [CTxOut(double_spend_value, CScript([b'a']))]
+        double_tx_hex = txToHex(double_tx)
+
+        try:
+            self.nodes[0].sendrawtransaction(double_tx_hex, True)
+        except JSONRPCException as exp:
+            assert_equal(exp.error['code'], -26)
+            assert_equal("too many potential replacements" in exp.error['message'], True)
+        else:
+            assert(False)
+
+        # If we remove an input, it should pass
+        double_tx = CTransaction()
+        double_tx.vin = inputs[0:-1]
+        double_tx.vout = [CTxOut(double_spend_value, CScript([b'a']))]
+        double_tx_hex = txToHex(double_tx)
+        self.nodes[0].sendrawtransaction(double_tx_hex, True)
+
+    def test_opt_in(self):
+        """ Replacing should only work if orig tx opted in """
+        tx0_outpoint = make_utxo(self.nodes[0], 1.1*COIN)
+
+        # Create a non-opting in transaction
+        tx1a = CTransaction()
+        tx1a.vin = [CTxIn(tx0_outpoint, nSequence=0xffffffff)]
+        tx1a.vout = [CTxOut(1*COIN, CScript([b'a']))]
+        tx1a_hex = txToHex(tx1a)
+        tx1a_txid = self.nodes[0].sendrawtransaction(tx1a_hex, True)
+
+        # Shouldn't be able to double-spend
+        tx1b = CTransaction()
+        tx1b.vin = [CTxIn(tx0_outpoint, nSequence=0)]
+        tx1b.vout = [CTxOut(0.9*COIN, CScript([b'b']))]
+        tx1b_hex = txToHex(tx1b)
+
+        try:
+            tx1b_txid = self.nodes[0].sendrawtransaction(tx1b_hex, True)
+        except JSONRPCException as exp:
+            assert_equal(exp.error['code'], -26)
+        else:
+            print tx1b_txid
+            assert(False)
+
+        tx1_outpoint = make_utxo(self.nodes[0], 1.1*COIN)
+
+        # Create a different non-opting in transaction
+        tx2a = CTransaction()
+        tx2a.vin = [CTxIn(tx1_outpoint, nSequence=0xfffffffe)]
+        tx2a.vout = [CTxOut(1*COIN, CScript([b'a']))]
+        tx2a_hex = txToHex(tx2a)
+        tx2a_txid = self.nodes[0].sendrawtransaction(tx2a_hex, True)
+
+        # Still shouldn't be able to double-spend
+        tx2b = CTransaction()
+        tx2b.vin = [CTxIn(tx1_outpoint, nSequence=0)]
+        tx2b.vout = [CTxOut(0.9*COIN, CScript([b'b']))]
+        tx2b_hex = txToHex(tx2b)
+
+        try:
+            tx2b_txid = self.nodes[0].sendrawtransaction(tx2b_hex, True)
+        except JSONRPCException as exp:
+            assert_equal(exp.error['code'], -26)
+        else:
+            assert(False)
+
+        # Now create a new transaction that spends from tx1a and tx2a
+        # opt-in on one of the inputs
+        # Transaction should be replaceable on either input
+
+        tx1a_txid = int(tx1a_txid, 16)
+        tx2a_txid = int(tx2a_txid, 16)
+
+        tx3a = CTransaction()
+        tx3a.vin = [CTxIn(COutPoint(tx1a_txid, 0), nSequence=0xffffffff),
+                    CTxIn(COutPoint(tx2a_txid, 0), nSequence=0xfffffffd)]
+        tx3a.vout = [CTxOut(0.9*COIN, CScript([b'c'])), CTxOut(0.9*COIN, CScript([b'd']))]
+        tx3a_hex = txToHex(tx3a)
+
+        self.nodes[0].sendrawtransaction(tx3a_hex, True)
+
+        tx3b = CTransaction()
+        tx3b.vin = [CTxIn(COutPoint(tx1a_txid, 0), nSequence=0)]
+        tx3b.vout = [CTxOut(0.5*COIN, CScript([b'e']))]
+        tx3b_hex = txToHex(tx3b)
+
+        tx3c = CTransaction()
+        tx3c.vin = [CTxIn(COutPoint(tx2a_txid, 0), nSequence=0)]
+        tx3c.vout = [CTxOut(0.5*COIN, CScript([b'f']))]
+        tx3c_hex = txToHex(tx3c)
+
+        self.nodes[0].sendrawtransaction(tx3b_hex, True)
+        # If tx3b was accepted, tx3c won't look like a replacement,
+        # but make sure it is accepted anyway
+        self.nodes[0].sendrawtransaction(tx3c_hex, True)
+
+if __name__ == '__main__':
+    ReplaceByFeeTest().main()
diff --git a/src/main.cpp b/src/main.cpp
index b0b58141a..e48d0a6d4 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -832,15 +832,42 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
         return state.Invalid(false, REJECT_ALREADY_KNOWN, "txn-already-in-mempool");
 
     // Check for conflicts with in-memory transactions
+    set<uint256> setConflicts;
     {
     LOCK(pool.cs); // protect pool.mapNextTx
-    for (unsigned int i = 0; i < tx.vin.size(); i++)
+    BOOST_FOREACH(const CTxIn &txin, tx.vin)
     {
-        COutPoint outpoint = tx.vin[i].prevout;
-        if (pool.mapNextTx.count(outpoint))
+        if (pool.mapNextTx.count(txin.prevout))
         {
-            // Disable replacement feature for now
-            return state.Invalid(false, REJECT_CONFLICT, "txn-mempool-conflict");
+            const CTransaction *ptxConflicting = pool.mapNextTx[txin.prevout].ptx;
+            if (!setConflicts.count(ptxConflicting->GetHash()))
+            {
+                // Allow opt-out of transaction replacement by setting
+                // nSequence >= maxint-1 on all inputs.
+                //
+                // maxint-1 is picked to still allow use of nLockTime by
+                // non-replacable transactions. All inputs rather than just one
+                // is for the sake of multi-party protocols, where we don't
+                // want a single party to be able to disable replacement.
+                //
+                // The opt-out ignores descendants as anyone relying on
+                // first-seen mempool behavior should be checking all
+                // unconfirmed ancestors anyway; doing otherwise is hopelessly
+                // insecure.
+                bool fReplacementOptOut = true;
+                BOOST_FOREACH(const CTxIn &txin, ptxConflicting->vin)
+                {
+                    if (txin.nSequence < std::numeric_limits<unsigned int>::max()-1)
+                    {
+                        fReplacementOptOut = false;
+                        break;
+                    }
+                }
+                if (fReplacementOptOut)
+                    return state.Invalid(false, REJECT_CONFLICT, "txn-mempool-conflict");
+
+                setConflicts.insert(ptxConflicting->GetHash());
+            }
         }
     }
     }
@@ -958,6 +985,160 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
             return state.DoS(0, false, REJECT_NONSTANDARD, "too-long-mempool-chain", false, errString);
         }
 
+        // A transaction that spends outputs that would be replaced by it is invalid. Now
+        // that we have the set of all ancestors we can detect this
+        // pathological case by making sure setConflicts and setAncestors don't
+        // intersect.
+        BOOST_FOREACH(CTxMemPool::txiter ancestorIt, setAncestors)
+        {
+            const uint256 &hashAncestor = ancestorIt->GetTx().GetHash();
+            if (setConflicts.count(hashAncestor))
+            {
+                return state.DoS(10, error("AcceptToMemoryPool: %s spends conflicting transaction %s",
+                                           hash.ToString(),
+                                           hashAncestor.ToString()),
+                                 REJECT_INVALID, "bad-txns-spends-conflicting-tx");
+            }
+        }
+
+        // Check if it's economically rational to mine this transaction rather
+        // than the ones it replaces.
+        CAmount nConflictingFees = 0;
+        size_t nConflictingSize = 0;
+        uint64_t nConflictingCount = 0;
+        CTxMemPool::setEntries allConflicting;
+
+        // If we don't hold the lock allConflicting might be incomplete; the
+        // subsequent RemoveStaged() and addUnchecked() calls don't guarantee
+        // mempool consistency for us.
+        LOCK(pool.cs);
+        if (setConflicts.size())
+        {
+            CFeeRate newFeeRate(nFees, nSize);
+            set<uint256> setConflictsParents;
+            const int maxDescendantsToVisit = 100;
+            CTxMemPool::setEntries setIterConflicting;
+            BOOST_FOREACH(const uint256 &hashConflicting, setConflicts)
+            {
+                CTxMemPool::txiter mi = pool.mapTx.find(hashConflicting);
+                if (mi == pool.mapTx.end())
+                    continue;
+
+                // Save these to avoid repeated lookups
+                setIterConflicting.insert(mi);
+
+                // If this entry is "dirty", then we don't have descendant
+                // state for this transaction, which means we probably have
+                // lots of in-mempool descendants.
+                // Don't allow replacements of dirty transactions, to ensure
+                // that we don't spend too much time walking descendants.
+                // This should be rare.
+                if (mi->IsDirty()) {
+                    return state.DoS(0,
+                            error("AcceptToMemoryPool: rejecting replacement %s; cannot replace tx %s with untracked descendants",
+                                hash.ToString(),
+                                mi->GetTx().GetHash().ToString()),
+                            REJECT_NONSTANDARD, "too many potential replacements");
+                }
+
+                // Don't allow the replacement to reduce the feerate of the
+                // mempool.
+                //
+                // We usually don't want to accept replacements with lower
+                // feerates than what they replaced as that would lower the
+                // feerate of the next block. Requiring that the feerate always
+                // be increased is also an easy-to-reason about way to prevent
+                // DoS attacks via replacements.
+                //
+                // The mining code doesn't (currently) take children into
+                // account (CPFP) so we only consider the feerates of
+                // transactions being directly replaced, not their indirect
+                // descendants. While that does mean high feerate children are
+                // ignored when deciding whether or not to replace, we do
+                // require the replacement to pay more overall fees too,
+                // mitigating most cases.
+                CFeeRate oldFeeRate(mi->GetFee(), mi->GetTxSize());
+                if (newFeeRate <= oldFeeRate)
+                {
+                    return state.DoS(0,
+                            error("AcceptToMemoryPool: rejecting replacement %s; new feerate %s <= old feerate %s",
+                                  hash.ToString(),
+                                  newFeeRate.ToString(),
+                                  oldFeeRate.ToString()),
+                            REJECT_INSUFFICIENTFEE, "insufficient fee");
+                }
+
+                BOOST_FOREACH(const CTxIn &txin, mi->GetTx().vin)
+                {
+                    setConflictsParents.insert(txin.prevout.hash);
+                }
+
+                nConflictingCount += mi->GetCountWithDescendants();
+            }
+            // This potentially overestimates the number of actual descendants
+            // but we just want to be conservative to avoid doing too much
+            // work.
+            if (nConflictingCount <= maxDescendantsToVisit) {
+                // If not too many to replace, then calculate the set of
+                // transactions that would have to be evicted
+                BOOST_FOREACH(CTxMemPool::txiter it, setIterConflicting) {
+                    pool.CalculateDescendants(it, allConflicting);
+                }
+                BOOST_FOREACH(CTxMemPool::txiter it, allConflicting) {
+                    nConflictingFees += it->GetFee();
+                    nConflictingSize += it->GetTxSize();
+                }
+            } else {
+                return state.DoS(0,
+                        error("AcceptToMemoryPool: rejecting replacement %s; too many potential replacements (%d > %d)\n",
+                            hash.ToString(),
+                            nConflictingCount,
+                            maxDescendantsToVisit),
+                        REJECT_NONSTANDARD, "too many potential replacements");
+            }
+
+            for (unsigned int j = 0; j < tx.vin.size(); j++)
+            {
+                // We don't want to accept replacements that require low
+                // feerate junk to be mined first. Ideally we'd keep track of
+                // the ancestor feerates and make the decision based on that,
+                // but for now requiring all new inputs to be confirmed works.
+                if (!setConflictsParents.count(tx.vin[j].prevout.hash))
+                {
+                    // Rather than check the UTXO set - potentially expensive -
+                    // it's cheaper to just check if the new input refers to a
+                    // tx that's in the mempool.
+                    if (pool.mapTx.find(tx.vin[j].prevout.hash) != pool.mapTx.end())
+                        return state.DoS(0, error("AcceptToMemoryPool: replacement %s adds unconfirmed input, idx %d",
+                                                  hash.ToString(), j),
+                                         REJECT_NONSTANDARD, "replacement-adds-unconfirmed");
+                }
+            }
+
+            // The replacement must pay greater fees than the transactions it
+            // replaces - if we did the bandwidth used by those conflicting
+            // transactions would not be paid for.
+            if (nFees < nConflictingFees)
+            {
+                return state.DoS(0, error("AcceptToMemoryPool: rejecting replacement %s, less fees than conflicting txs; %s < %s",
+                                          hash.ToString(), FormatMoney(nFees), FormatMoney(nConflictingFees)),
+                                 REJECT_INSUFFICIENTFEE, "insufficient fee");
+            }
+
+            // Finally in addition to paying more fees than the conflicts the
+            // new transaction must pay for its own bandwidth.
+            CAmount nDeltaFees = nFees - nConflictingFees;
+            if (nDeltaFees < ::minRelayTxFee.GetFee(nSize))
+            {
+                return state.DoS(0,
+                        error("AcceptToMemoryPool: rejecting replacement %s, not enough additional fees to relay; %s < %s",
+                              hash.ToString(),
+                              FormatMoney(nDeltaFees),
+                              FormatMoney(::minRelayTxFee.GetFee(nSize))),
+                        REJECT_INSUFFICIENTFEE, "insufficient fee");
+            }
+        }
+
         // Check against previous transactions
         // This is done last to help prevent CPU exhaustion denial-of-service attacks.
         if (!CheckInputs(tx, state, view, true, STANDARD_SCRIPT_VERIFY_FLAGS, true))
@@ -978,6 +1159,17 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
                 __func__, hash.ToString(), FormatStateMessage(state));
         }
 
+        // Remove conflicting transactions from the mempool
+        BOOST_FOREACH(const CTxMemPool::txiter it, allConflicting)
+        {
+            LogPrint("mempool", "replacing tx %s with %s for %s BTC additional fees, %d delta bytes\n",
+                    it->GetTx().GetHash().ToString(),
+                    hash.ToString(),
+                    FormatMoney(nFees - nConflictingFees),
+                    (int)nSize - (int)nConflictingSize);
+        }
+        pool.RemoveStaged(allConflicting);
+
         // Store transaction in memory
         pool.addUnchecked(hash, entry, setAncestors, !IsInitialBlockDownload());
 
diff --git a/src/txmempool.h b/src/txmempool.h
index 5d8231fb7..7f43120f7 100644
--- a/src/txmempool.h
+++ b/src/txmempool.h
@@ -420,6 +420,11 @@ public:
      */
     bool CalculateMemPoolAncestors(const CTxMemPoolEntry &entry, setEntries &setAncestors, uint64_t limitAncestorCount, uint64_t limitAncestorSize, uint64_t limitDescendantCount, uint64_t limitDescendantSize, std::string &errString, bool fSearchForParents = true);
 
+    /** Populate setDescendants with all in-mempool descendants of hash.
+     *  Assumes that setDescendants includes all in-mempool descendants of anything
+     *  already in it.  */
+    void CalculateDescendants(txiter it, setEntries &setDescendants);
+
     /** The minimum fee to get into the mempool, which may itself not be enough
       *  for larger-sized transactions.
       *  The minReasonableRelayFee constructor arg is used to bound the time it
@@ -505,10 +510,6 @@ private:
     void UpdateForRemoveFromMempool(const setEntries &entriesToRemove);
     /** Sever link between specified transaction and direct children. */
     void UpdateChildrenForRemoval(txiter entry);
-    /** Populate setDescendants with all in-mempool descendants of hash.
-     *  Assumes that setDescendants includes all in-mempool descendants of anything
-     *  already in it.  */
-    void CalculateDescendants(txiter it, setEntries &setDescendants);
 
     /** Before calling removeUnchecked for a given transaction,
      *  UpdateForRemoveFromMempool must be called on the entire (dependent) set