diff --git a/interpreter/CppInterOp/unittests/CppInterOp/InterpreterTest.cpp b/interpreter/CppInterOp/unittests/CppInterOp/InterpreterTest.cpp
index eb08cbf816efd..ed906a49cd5f4 100644
--- a/interpreter/CppInterOp/unittests/CppInterOp/InterpreterTest.cpp
+++ b/interpreter/CppInterOp/unittests/CppInterOp/InterpreterTest.cpp
@@ -853,6 +853,7 @@ TYPED_TEST(CPPINTEROP_TEST_MODE, Interpreter_DetectSystemCompilerIncludePaths) {
 TYPED_TEST(CPPINTEROP_TEST_MODE, Interpreter_IncludePaths) {
   if (TypeParam::isOutOfProcess)
     GTEST_SKIP() << "Test fails for OOP JIT builds";
+  TestFixture::CreateInterpreter();
   std::vector<std::string> includes;
   Cpp::GetIncludePaths(includes);
   EXPECT_FALSE(includes.empty());
@@ -955,7 +956,8 @@ TYPED_TEST(CPPINTEROP_TEST_MODE, Interpreter_GetLanguageStandardGNU) {
 }
 
 TYPED_TEST(CPPINTEROP_TEST_MODE, Interpreter_ExternalInterpreter) {
-
+  // UseExternalInterpreter must refuse while an interpreter is registered.
+  TestFixture::CreateInterpreter();
 
 #ifndef CPPINTEROP_USE_CLING
   llvm::ExitOnError ExitOnErr;
diff --git a/interpreter/CppInterOp/unittests/CppInterOp/ScopeReflectionTest.cpp b/interpreter/CppInterOp/unittests/CppInterOp/ScopeReflectionTest.cpp
index 749b6eba053c3..2fc5fdc38ef8e 100644
--- a/interpreter/CppInterOp/unittests/CppInterOp/ScopeReflectionTest.cpp
+++ b/interpreter/CppInterOp/unittests/CppInterOp/ScopeReflectionTest.cpp
@@ -781,6 +781,7 @@ TYPED_TEST(CPPINTEROP_TEST_MODE, ScopeReflection_GetUsingNamespaces) {
 }
 
 TYPED_TEST(CPPINTEROP_TEST_MODE, ScopeReflection_GetGlobalScope) {
+  TestFixture::CreateInterpreter();
   EXPECT_EQ(Cpp::GetQualifiedName(Cpp::GetGlobalScope()), "");
   EXPECT_EQ(Cpp::GetName(Cpp::GetGlobalScope()), "");
 }
diff --git a/interpreter/CppInterOp/unittests/CppInterOp/Utils.cpp b/interpreter/CppInterOp/unittests/CppInterOp/Utils.cpp
index 2192dd4a69d03..885b961f119d0 100644
--- a/interpreter/CppInterOp/unittests/CppInterOp/Utils.cpp
+++ b/interpreter/CppInterOp/unittests/CppInterOp/Utils.cpp
@@ -44,6 +44,22 @@ bool HasRuntimeModulemaps() {
 }
 }
 
+namespace {
+// Interpreters do not outlive the test that created them: kept alive, they
+// pile up for the whole run and exhaust a 32-bit address space.
+class DeleteInterpretersOnTestEnd : public ::testing::EmptyTestEventListener {
+  void OnTestEnd(const ::testing::TestInfo& /*info*/) override {
+    while (Cpp::DeleteInterpreter())
+      ;
+  }
+};
+[[maybe_unused]] const bool kDeleteInterpretersOnTestEnd = [] {
+  ::testing::UnitTest::GetInstance()->listeners().Append(
+      new DeleteInterpretersOnTestEnd);
+  return true;
+}();
+} // namespace
+
 void TestUtils::GetAllTopLevelDecls(
     const std::string& code, std::vector<Decl*>& Decls,
     bool filter_implicitGenerated /* = false */,