diff --git a/src/test/validation_tests.cpp b/src/test/validation_tests.cpp
--- a/src/test/validation_tests.cpp
+++ b/src/test/validation_tests.cpp
@@ -1,5 +1,6 @@
 // Copyright (c) 2011-2016 The Bitcoin Core developers
 // Copyright (c) 2017 The Bitcoin developers
+// Copyright (c) 2018 The Bitcoin Cash developers
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
@@ -7,6 +8,7 @@
 #include "config.h"
 #include "consensus/consensus.h"
 #include "primitives/transaction.h"
+#include "script/interpreter.h"
 #include "test/test_bitcoin.h"
 #include "util.h"
 #include "validation.h"
@@ -17,6 +19,12 @@
 
 #include <boost/test/unit_test.hpp>
 
+// defined in validation.cpp but not declared in validation.h - dont want to
+// expose in header just for tests
+// extern uint32_t GetBlockScriptFlags(const CBlockIndex *pindex, const Config
+// &config);
+uint32_t GetBlockScriptFlags(const CBlockIndex *pindex, const Config &config);
+
 static CBlock makeLargeDummyBlock(const size_t num_tx) {
     CBlock block;
     block.vtx.reserve(num_tx);
@@ -76,4 +84,32 @@
     BOOST_CHECK_NO_THROW({ LoadExternalBlockFile(config, fp, 0); });
 }
 
+/** test that GetBlockScriptFlags produces the correct flags for monolith
+ * upgrade (May 2018) */
+BOOST_AUTO_TEST_CASE(getblockscriptflags_monolith) {
+    GlobalConfig config;
+
+    // add a block to the chain - chain will be (genesis block, new block)
+    CBlockIndex newBlock;
+    CBlockIndex *genesisBlock = chainActive.Tip();
+    newBlock.pprev = genesisBlock;
+    chainActive.SetTip(&newBlock);
+
+    // monolith not enabled
+    BOOST_CHECK(!IsMonolithEnabled(config, genesisBlock));
+
+    uint32_t flags = GetBlockScriptFlags(&newBlock, config);
+    BOOST_CHECK((flags & SCRIPT_ENABLE_OPCODES_MONOLITH) == 0);
+
+    // Activate May 15, 2018 HF the dirty way
+    const int64_t monolithTime =
+        config.GetChainParams().GetConsensus().monolithActivationTime;
+    genesisBlock->nTime = monolithTime;
+
+    BOOST_CHECK(IsMonolithEnabled(config, genesisBlock));
+
+    flags = GetBlockScriptFlags(&newBlock, config);
+    BOOST_CHECK((flags & SCRIPT_ENABLE_OPCODES_MONOLITH) != 0);
+}
+
 BOOST_AUTO_TEST_SUITE_END()
diff --git a/src/validation.cpp b/src/validation.cpp
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -1,6 +1,7 @@
 // Copyright (c) 2009-2010 Satoshi Nakamoto
 // Copyright (c) 2009-2016 The Bitcoin Core developers
 // Copyright (c) 2017 The Bitcoin developers
+// Copyright (c) 2018 The Bitcoin Cash developers
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
@@ -195,8 +196,7 @@
 static void FindFilesToPrune(std::set<int> &setFilesToPrune,
                              uint64_t nPruneAfterHeight);
 static FILE *OpenUndoFile(const CDiskBlockPos &pos, bool fReadOnly = false);
-static uint32_t GetBlockScriptFlags(const CBlockIndex *pindex,
-                                    const Config &config);
+uint32_t GetBlockScriptFlags(const CBlockIndex *pindex, const Config &config);
 
 static bool IsFinalTx(const CTransaction &tx, int nBlockHeight,
                       int64_t nBlockTime) {
@@ -1847,8 +1847,7 @@
 static ThresholdConditionCache warningcache[VERSIONBITS_NUM_BITS];
 
 // Returns the script flags which should be checked for a given block
-static uint32_t GetBlockScriptFlags(const CBlockIndex *pindex,
-                                    const Config &config) {
+uint32_t GetBlockScriptFlags(const CBlockIndex *pindex, const Config &config) {
     AssertLockHeld(cs_main);
     const Consensus::Params &consensusparams =
         config.GetChainParams().GetConsensus();
@@ -1892,6 +1891,11 @@
         flags |= SCRIPT_VERIFY_NULLFAIL;
     }
 
+    if (IsMonolithEnabled(config, pindex->pprev)) {
+        // When the May 15, 2018 HF is enabled, activate new opcodes.
+        flags |= SCRIPT_ENABLE_OPCODES_MONOLITH;
+    }
+
     return flags;
 }