diff --git a/test/functional/bip68-sequence.py b/test/functional/bip68-sequence.py
--- a/test/functional/bip68-sequence.py
+++ b/test/functional/bip68-sequence.py
@@ -224,13 +224,18 @@
         tx1 = FromHex(CTransaction(), self.nodes[0].getrawtransaction(txid))
         tx1.rehash()
 
+        # Clear the existing mempool into a block.
+        self.nodes[0].generate(1)
+
         # Anyone-can-spend mempool tx.
         # Sequence lock of 0 should pass.
         tx2 = CTransaction()
         tx2.nVersion = 2
         tx2.vin = [CTxIn(COutPoint(tx1.sha256, 0), nSequence=0)]
         tx2.vout = [
-            CTxOut(int(tx1.vout[0].nValue - self.relayfee * COIN), CScript([b'a']))]
+            CTxOut(int(0), CScript([b'a']))]
+        tx2.vout[0].nValue = tx1.vout[0].nValue - \
+            2 * self.nodes[0].calculate_fee(tx2)
         tx2_raw = self.nodes[0].signrawtransaction(ToHex(tx2))["hex"]
         tx2 = FromHex(tx2, tx2_raw)
         tx2.rehash()
@@ -250,7 +255,7 @@
             tx.vin = [
                 CTxIn(COutPoint(orig_tx.sha256, 0), nSequence=sequence_value)]
             tx.vout = [
-                CTxOut(int(orig_tx.vout[0].nValue - relayfee * COIN), CScript([b'a']))]
+                CTxOut(int(orig_tx.vout[0].nValue - 2 * self.nodes[0].calculate_fee(tx)), CScript([b'a']))]
             tx.rehash()
 
             if (orig_tx.hash in node.getrawmempool()):
@@ -263,6 +268,8 @@
 
             return tx
 
+        self.log.info("mempool size {}".format(
+            len(self.nodes[0].getrawmempool())))
         test_nonzero_locks(
             tx2, self.nodes[0], self.relayfee, use_height_lock=True)
         test_nonzero_locks(
@@ -271,7 +278,7 @@
         # Now mine some blocks, but make sure tx2 doesn't get mined.
         # Use prioritisetransaction to lower the effective feerate to 0
         self.nodes[0].prioritisetransaction(
-            tx2.hash, -1e15, int(-self.relayfee * COIN))
+            tx2.hash, -1e15, -2 * self.nodes[0].calculate_fee(tx2))
         cur_time = int(time.time())
         for i in range(10):
             self.nodes[0].setmocktime(cur_time + 600)
@@ -287,7 +294,7 @@
 
         # Mine tx2, and then try again
         self.nodes[0].prioritisetransaction(
-            tx2.hash, 1e15, self.nodes[0].calculate_fee(tx2))
+            tx2.hash, 1e15, 100 * self.nodes[0].calculate_fee(tx2))
 
         # Advance the time on the node so that we can test timelocks
         self.nodes[0].setmocktime(cur_time + 600)