diff --git a/Core/HLE/sceKernelThread.cpp b/Core/HLE/sceKernelThread.cpp
index 7056b5ff8..2e1293e58 100644
--- a/Core/HLE/sceKernelThread.cpp
+++ b/Core/HLE/sceKernelThread.cpp
@@ -150,7 +150,9 @@ public:
 		p.Do(savedRA);
 		p.Do(savedV0);
 		p.Do(savedV1);
-		p.Do(savedIdRegister);
+		// No longer used.
+		u32 legacySavedIdRegister = 0;
+		p.Do(legacySavedIdRegister);
 	}
 
 	NativeCallback nc;
@@ -159,7 +161,6 @@ public:
 	u32 savedRA;
 	u32 savedV0;
 	u32 savedV1;
-	u32 savedIdRegister;
 };
 
 #if COMMON_LITTLE_ENDIAN
@@ -944,7 +945,9 @@ void MipsCall::DoState(PointerWrap &p)
 	p.Do(cbId);
 	p.DoArray(args, ARRAY_SIZE(args));
 	p.Do(numArgs);
-	p.Do(savedIdRegister);
+	// No longer used.
+	u32 legacySavedIdRegister = 0;
+	p.Do(legacySavedIdRegister);
 	p.Do(savedRa);
 	p.Do(savedPc);
 	p.Do(savedV0);
@@ -3356,16 +3359,12 @@ void __KernelExecuteMipsCallOnCurrentThread(u32 callId, bool reschedAfter)
 	call->savedRa = currentMIPS->r[MIPS_REG_RA];
 	call->savedV0 = currentMIPS->r[MIPS_REG_V0];
 	call->savedV1 = currentMIPS->r[MIPS_REG_V1];
-	call->savedIdRegister = currentMIPS->r[MIPS_REG_CALL_ID];
 	call->savedId = cur->currentMipscallId;
 	call->reschedAfter = reschedAfter;
 
 	// Set up the new state
 	currentMIPS->pc = call->entryPoint;
 	currentMIPS->r[MIPS_REG_RA] = __KernelMipsCallReturnAddress();
-	// We put this two places in case the game overwrites it.
-	// We may want it later to "inject" return values.
-	currentMIPS->r[MIPS_REG_CALL_ID] = callId;
 	cur->currentMipscallId = callId;
 	for (int i = 0; i < call->numArgs; i++) {
 		currentMIPS->r[MIPS_REG_A0 + i] = call->args[i];
@@ -3388,9 +3387,6 @@ void __KernelReturnFromMipsCall()
 	}
 
 	u32 callId = cur->currentMipscallId;
-	if (currentMIPS->r[MIPS_REG_CALL_ID] != callId)
-		WARN_LOG_REPORT(SCEKERNEL, "__KernelReturnFromMipsCall(): s0 is %08x != %08x", currentMIPS->r[MIPS_REG_CALL_ID], callId);
-
 	MipsCall *call = mipsCalls.pop(callId);
 
 	// Value returned by the callback function
@@ -3408,7 +3404,6 @@ void __KernelReturnFromMipsCall()
 	currentMIPS->r[MIPS_REG_RA] = call->savedRa;
 	currentMIPS->r[MIPS_REG_V0] = call->savedV0;
 	currentMIPS->r[MIPS_REG_V1] = call->savedV1;
-	currentMIPS->r[MIPS_REG_CALL_ID] = call->savedIdRegister;
 	cur->currentMipscallId = call->savedId;
 
 	if (call->cbId != 0)
diff --git a/Core/HLE/sceKernelThread.h b/Core/HLE/sceKernelThread.h
index a73d6ab81..17bc0da9e 100644
--- a/Core/HLE/sceKernelThread.h
+++ b/Core/HLE/sceKernelThread.h
@@ -251,7 +251,6 @@ struct MipsCall {
 	u32 args[6];
 	int numArgs;
 	Action *doAfter;
-	u32 savedIdRegister;
 	u32 savedRa;
 	u32 savedPc;
 	u32 savedV0;
diff --git a/Core/MIPS/MIPS.h b/Core/MIPS/MIPS.h
index 4e1f20148..fd9c829d2 100644
--- a/Core/MIPS/MIPS.h
+++ b/Core/MIPS/MIPS.h
@@ -63,8 +63,6 @@ enum MIPSGPReg
 	MIPS_REG_FP=30,
 	MIPS_REG_RA=31,
 
-	// ID for mipscall "callback" is stored here - from JPCSP
-	MIPS_REG_CALL_ID=MIPS_REG_S0,
 	MIPS_REG_INVALID=-1,
 
 	// Not real regs, just for convenience/jit mapping.