Hi help needed;

I've started work on a form with multiple combo boxes which I would like users to be able to search. I've got tables as row sources for the combo boxes to limit users' choices, however I want the users to be able to leave some combo boxes blank if they so wish, so searches can be performed for all the records in that particular field. The problem I'm having is how to give user's an option of leaving a combo box blank. I can't put an empty value in the rowsource table as the field is a primary key.
Does this make sense? Can anybody help?

Couldn't you just have the user not select anything if they want to search for all??? Then you could code the search button by saying:

if isnull(me("cmbChoices")) then
'Search for All!
end if

HTH

Doug