diff --git a/Lib/test/support.py b/Lib/test/support.py
--- a/Lib/test/support.py
+++ b/Lib/test/support.py
@@ -198,6 +198,76 @@
         return fresh_module
 
 
+def _swap_out_module(test_globals, modname, module, names):
+    # XXX ensure all the targets already exist?
+    sys.modules[modname] = module
+    test_globals[modname] = module
+    for name in names:
+        test_globals[name] = getattr(module, name)
+
+
+def conforms_to_pep399(test_globals, module, c_module_name, names=None):
+    """A class decorator that makes a test case conform to PEP 399.
+
+    Two new subclasses of the decorated class are created and the pair
+    is returned instead of the original class.  Both are also added to
+    the globals.
+
+    The following happens for the duration of each subclass's tests:
+
+    * the module is replaced in sys.modules with the C or pure Python
+      version,
+    * the module is replaced in the globals,
+    * any of the "names" found in the globals are replaced by the
+      corresponding module attribute.
+
+    """
+    modname = module.__name__
+
+    def decorator(cls):
+        if unittest.TestCase not in cls.__bases__:
+            bases = (cls, unittest.TestCase)
+        else:
+            bases = (cls,)
+
+        pymod = import_fresh_module(modname, blocked=[c_module_name])
+        cmod = import_fresh_module(modname, fresh=[c_module_name])
+        cskipmsg = 'requires the C version of the {} module'.format(modname)
+
+        class PurePythonTests(*bases):
+            @classmethod
+            def setUpClass(cls):
+                _swap_out_module(test_globals, modname, pymod, names)
+                super(PurePythonTests, cls).setUpClass()
+            @classmethod
+            def tearDownClass(cls):
+                _swap_out_module(test_globals, modname, module, names)
+                super(PurePythonTests, cls).tearDownClass()
+        PurePythonTests.__name__ = "PurePython" + cls.__name__
+        PurePythonTests.__module__ = test_globals['__name__']
+        test_globals[PurePythonTests.__name__] = PurePythonTests
+
+        if cmod:
+            @unittest.skipUnless(cmod, cskipmsg)
+            class CPythonTests(*bases):
+                @classmethod
+                def setUpClass(cls):
+                    _swap_out_module(test_globals, modname, cmod, names)
+                    super(CPythonTests, cls).setUpClass()
+                @classmethod
+                def tearDownClass(cls):
+                    _swap_out_module(test_globals, modname, module, names)
+                    super(CPythonTests, cls).tearDownClass()
+            CPythonTests.__name__ = "CPython" + cls.__name__
+            CPythonTests.__module__ = test_globals['__name__']
+            test_globals[CPythonTests.__name__] = CPythonTests
+        else:
+            CPythonTests = None
+
+        return (PurePythonTests, CPythonTests)
+    return decorator
+
+
 def get_attribute(obj, name):
     """Get an attribute, raising SkipTest if AttributeError is raised."""
     try:
diff --git a/Lib/test/test_support.py b/Lib/test/test_support.py
--- a/Lib/test/test_support.py
+++ b/Lib/test/test_support.py
@@ -26,6 +26,16 @@
     def test_import_fresh_module(self):
         support.import_fresh_module("ftplib")
 
+    def test_conforms_to_pep_399(self):
+        @support.conforms_to_pep399(globals(), support, '_support', ['TESTFN'])
+        class SomeTest:
+            def test_something(self):
+                self.assertNotEqual('spam', 'ham')
+
+        self.assertIn('PurePythonSomeTest', globals())
+        # clean up a side-effect
+        del globals()['PurePythonSomeTest']
+
     def test_get_attribute(self):
         self.assertEqual(support.get_attribute(self, "test_get_attribute"),
                         self.test_get_attribute)