diff --git a/Lib/test/test_typechecks.py b/Lib/test/test_typechecks.py
new file mode 100644
index 0000000..632598c
--- /dev/null
+++ b/Lib/test/test_typechecks.py
@@ -0,0 +1,75 @@
+"""Unit tests for __instancecheck__ and __subclasscheck__."""
+
+import unittest
+from test import test_support
+
+
+class ABC(type):
+
+    def __instancecheck__(cls, inst):
+        """Implement isinstance(inst, cls)."""
+        return any(cls.__subclasscheck__(c)
+                   for c in {type(inst), inst.__class__})
+
+    def __subclasscheck__(cls, sub):
+        """Implement issubclass(sub, cls)."""
+        candidates = cls.__dict__.get("__subclass__", set()) | {cls}
+        return any(c in candidates for c in sub.mro())
+
+
+class Integer(metaclass=ABC):
+
+    __subclass__ = {int}
+
+
+class SubInt(Integer):
+
+    pass
+
+
+class Evil:
+    def __instancecheck__(self, inst): return False
+
+
+class TypeChecksTest(unittest.TestCase):
+
+    def testIsSubclassInternal(self):
+        self.assertEqual(Integer.__subclasscheck__(int), True)
+        self.assertEqual(Integer.__subclasscheck__(float), False)
+
+    def testIsSubclassBuiltin(self):
+        self.assertEqual(issubclass(int, Integer), True)
+        self.assertEqual(issubclass(float, Integer), False)
+
+    def testIsInstanceBuiltin(self):
+        self.assertEqual(isinstance(42, Integer), True)
+        self.assertEqual(isinstance(3.14, Integer), False)
+
+    def testIsInstanceActual(self):
+        self.assertEqual(isinstance(Integer(), Integer), True)
+
+    def testIsSubclassActual(self):
+        self.assertEqual(issubclass(Integer, Integer), True)
+
+    def testSubclassBehavior(self):
+        self.assertEqual(issubclass(SubInt, Integer), True)
+        self.assertEqual(issubclass(SubInt, SubInt), True)
+        self.assertEqual(issubclass(Integer, SubInt), False)
+        self.assertEqual(issubclass(int, SubInt), False)
+        self.assertEqual(isinstance(SubInt(), Integer), True)
+        self.assertEqual(isinstance(SubInt(), SubInt), True)
+        self.assertEqual(isinstance(42, SubInt), False)
+
+    def testInfiniteRecursionCaughtProperly(self):
+        e = Evil()
+        # This invokes isinstance() recursively, until the stack is exhausted.
+        self.assertRaises(RuntimeError, isinstance, e, Evil)
+        # XXX How to check the same situation for issubclass()?
+
+
+def test_main():
+    test_support.run_unittest(TypeChecksTest)
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/Objects/abstract.c b/Objects/abstract.c
index 645fd95..dc66893 100644
--- a/Objects/abstract.c
+++ b/Objects/abstract.c
@@ -2130,7 +2130,25 @@
 int
 PyObject_IsInstance(PyObject *inst, PyObject *cls)
 {
-    return recursive_isinstance(inst, cls, Py_GetRecursionLimit());
+	PyObject *t, *v, *tb;
+	PyErr_Fetch(&t, &v, &tb);
+	PyObject *checker = PyObject_GetAttrString(cls, "__instancecheck__");
+	PyErr_Restore(t, v, tb);
+	if (checker != NULL) {
+		PyObject *res;
+		int ok = -1;
+		if (Py_EnterRecursiveCall(" in __instancecheck__"))
+			return ok;
+		res = PyObject_CallFunctionObjArgs(checker, inst, NULL);
+		Py_LeaveRecursiveCall();
+		Py_DECREF(checker);
+		if (res != NULL) {
+			ok = PyObject_IsTrue(res);
+			Py_DECREF(res);
+		}
+		return ok;
+	}
+	return recursive_isinstance(inst, cls, Py_GetRecursionLimit());
 }
 
 static  int
@@ -2180,7 +2198,25 @@
 int
 PyObject_IsSubclass(PyObject *derived, PyObject *cls)
 {
-    return recursive_issubclass(derived, cls, Py_GetRecursionLimit());
+	PyObject *t, *v, *tb;
+	PyErr_Fetch(&t, &v, &tb);
+	PyObject *checker = PyObject_GetAttrString(cls, "__subclasscheck__");
+	PyErr_Restore(t, v, tb);
+	if (checker != NULL) {
+		PyObject *res;
+		int ok = -1;
+		if (Py_EnterRecursiveCall(" in __subclasscheck__"))
+			return ok;
+		res = PyObject_CallFunctionObjArgs(checker, derived, NULL);
+		Py_LeaveRecursiveCall();
+		Py_DECREF(checker);
+		if (res != NULL) {
+			ok = PyObject_IsTrue(res);
+			Py_DECREF(res);
+		}
+		return ok;
+	}
+	return recursive_issubclass(derived, cls, Py_GetRecursionLimit());
 }