Index: django/db/models/fields/related.py
===================================================================
--- django/db/models/fields/related.py	(revision 4429)
+++ django/db/models/fields/related.py	(working copy)
@@ -316,18 +316,20 @@
             # join_table: name of the m2m link table
             # source_col_name: the PK colname in join_table for the source object
             # target_col_name: the PK colname in join_table for the target object
-            # *objs - objects to add
+            # *objs - objects to add. Either object instances, or primary keys of object instances.
             from django.db import connection
 
             # If there aren't any objects, there is nothing to do.
             if objs:
                 # Check that all the objects are of the right type
+                new_ids = set()
                 for obj in objs:
-                    if not isinstance(obj, self.model):
-                        raise ValueError, "objects to add() must be %s instances" % self.model._meta.object_name
+                    if isinstance(obj, self.model):
+                        new_ids.add(obj._get_pk_val())
+                    else:
+                        new_ids.add(obj)
                 # Add the newly created or already existing objects to the join table.
                 # First find out which items are already added, to avoid adding them twice
-                new_ids = set([obj._get_pk_val() for obj in objs])
                 cursor = connection.cursor()
                 cursor.execute("SELECT %s FROM %s WHERE %s = %%s AND %s IN (%s)" % \
                     (target_col_name, self.join_table, source_col_name,
@@ -354,11 +356,13 @@
             # If there aren't any objects, there is nothing to do.
             if objs:
                 # Check that all the objects are of the right type
+                old_ids = set()
                 for obj in objs:
-                    if not isinstance(obj, self.model):
-                        raise ValueError, "objects to remove() must be %s instances" % self.model._meta.object_name
+                    if isinstance(obj, self.model):
+                        old_ids.add(obj._get_pk_val())
+                    else:
+                        old_ids.add(obj)
                 # Remove the specified objects from the join table
-                old_ids = set([obj._get_pk_val() for obj in objs])
                 cursor = connection.cursor()
                 cursor.execute("DELETE FROM %s WHERE %s = %%s AND %s IN (%s)" % \
                     (self.join_table, source_col_name,
Index: tests/modeltests/many_to_many/models.py
===================================================================
--- tests/modeltests/many_to_many/models.py	(revision 4427)
+++ tests/modeltests/many_to_many/models.py	(working copy)
@@ -203,7 +203,19 @@
 >>> p2.article_set.all()
 [<Article: Oxygen-free diet works wonders>]
 
-# Recreate the article and Publication we just deleted.
+# Relation sets can also be set using primary key values
+>>> p2.article_set = [a4.id, a5.id]
+>>> p2.article_set.all()
+[<Article: NASA finds intelligent life on Earth>, <Article: Oxygen-free diet works wonders>]
+>>> a4.publications.all()
+[<Publication: Science News>]
+>>> a4.publications = [p3.id]
+>>> p2.article_set.all()
+[<Article: Oxygen-free diet works wonders>]
+>>> a4.publications.all()
+[<Publication: Science Weekly>]
+
+# Recreate the article and Publication we have deleted.
 >>> p1 = Publication(id=None, title='The Python Journal')
 >>> p1.save()
 >>> a2 = Article(id=None, headline='NASA uses Python')