Access not enforcing referential integrity!!!

Howdy.

Using Acc 2007. Have two tables related one-to-many. Ref. integrity being
enforced, without any cascading updates or deletes. Data in both tables.

Problem: I can add new records to the many-side table, without any value for
the foreign key, and the new record is created without error or prompt from
Access. I can do this both in the child table directly, and using a
form/subform.

On the other hand, if I create a new child record and enter a FK value that
doesn't exist in the parent table, I get a message saying that a related
record in the parent table is required, and Access prohibits creation of
orphan.

What the heck is going on?