diff --git a/core/src/test/java/com/google/bitcoin/core/BlockChainTest.java b/core/src/test/java/com/google/bitcoin/core/BlockChainTest.java
index 6190ae18..5e14e76b 100644
--- a/core/src/test/java/com/google/bitcoin/core/BlockChainTest.java
+++ b/core/src/test/java/com/google/bitcoin/core/BlockChainTest.java
@@ -183,8 +183,7 @@ public class BlockChainTest {
         assertTrue(testNetChain.add(getBlock1()));
         Block b2 = getBlock2();
         assertTrue(testNetChain.add(b2));
-        NetworkParameters params2 = NetworkParameters.testNet();
-        Block bad = new Block(params2);
+        Block bad = new Block(testNet);
         // Merkle root can be anything here, doesn't matter.
         bad.setMerkleRoot(new Sha256Hash("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"));
         // Nonce was just some number that made the hash < difficulty limit set below, it can be anything.
@@ -205,7 +204,8 @@ public class BlockChainTest {
         }
 
         // Accept any level of difficulty now.
-        params2.proofOfWorkLimit = new BigInteger
+        BigInteger oldVal = testNet.proofOfWorkLimit;
+        testNet.proofOfWorkLimit = new BigInteger
                 ("00ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff", 16);
         try {
             testNetChain.add(bad);
@@ -214,6 +214,7 @@ public class BlockChainTest {
         } catch (VerificationException e) {
             assertTrue(e.getMessage(), e.getCause().getMessage().indexOf("Unexpected change in difficulty") >= 0);
         }
+        testNet.proofOfWorkLimit = oldVal;
 
         // TODO: Test difficulty change is not out of range when a transition period becomes valid.
     }
diff --git a/core/src/test/java/com/google/bitcoin/core/FullPrunedBlockChainTest.java b/core/src/test/java/com/google/bitcoin/core/FullPrunedBlockChainTest.java
index 1ddcec9b..33ef4ef4 100644
--- a/core/src/test/java/com/google/bitcoin/core/FullPrunedBlockChainTest.java
+++ b/core/src/test/java/com/google/bitcoin/core/FullPrunedBlockChainTest.java
@@ -21,6 +21,7 @@ import com.google.bitcoin.core.Transaction.SigHash;
 import com.google.bitcoin.store.FullPrunedBlockStore;
 import com.google.bitcoin.store.MemoryFullPrunedBlockStore;
 import com.google.bitcoin.utils.BriefLogFormatter;
+import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;
 import org.slf4j.Logger;
@@ -48,15 +49,23 @@ public class FullPrunedBlockChainTest {
     private FullPrunedBlockChain chain;
     private FullPrunedBlockStore store;
 
+    private int oldInterval;
+
     @Before
     public void setUp() throws Exception {
         BriefLogFormatter.init();
         unitTestParams = NetworkParameters.unitTests();
+        oldInterval = unitTestParams.interval;
         unitTestParams.interval = 10000;
         
         store = new MemoryFullPrunedBlockStore(unitTestParams, UNDOABLE_BLOCKS_STORED);
         chain = new FullPrunedBlockChain(unitTestParams, store);
     }
+
+    @After
+    public void tearDown() {
+        unitTestParams.interval = oldInterval;
+    }
     
     @Test
     public void testGeneratedChain() throws Exception {