<OK, I have a combobox on a form. Is there a way to ensure that once an item is selected from the combobox, it cannot be selected again in another record - the item is unique.>

Your advice was great. Hvr, there is a slight prob...
The form concerned is assigned to one table (students), and the combo box takes data from another table (books). How do I get the check box "Used" to link to the "books" table?

If I understand it, your have a form that books can be assigned to students. Once a book is assign to one student it can't be assigned to another student. Place the "Used" Yes/No field in with the books table and create a query on the books table to be used as the recordsource for your combobox. Everytime you have a new record the query will be run and the previously assigned books will not be shown in your combo box.

Thanx Carol, I greatly appreciate it!