diff --git a/src/consensus/validation.h b/src/consensus/validation.h
--- a/src/consensus/validation.h
+++ b/src/consensus/validation.h
@@ -16,6 +16,7 @@
 static const uint8_t REJECT_NONSTANDARD = 0x40;
 static const uint8_t REJECT_INSUFFICIENTFEE = 0x42;
 static const uint8_t REJECT_CHECKPOINT = 0x43;
+static const uint8_t REJECT_NOT_READY = 0x44;
 
 /** Capture information about block/transaction validation */
 class CValidationState {
diff --git a/src/validation.cpp b/src/validation.cpp
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -550,6 +550,12 @@
     const Consensus::Params &consensusParams =
         config.GetChainParams().GetConsensus();
 
+    if (!IsUAHFenabled(consensusParams, chainActive.Tip())) {
+        return state.DoS(0, false, REJECT_NOT_READY, "txn-too-early",
+                         "Cannot accept transactions into mempool while chain "
+                         "is before UAHF.");
+    }
+
     const CTransaction &tx = *ptx;
     const TxId txid = tx.GetId();