How to limit the number of choices in multiple drop-down lists?

[Stefan48]

Long story short, let's assume that I have a drop-down list in every cell of the range A1:B4 - they offer the same choices, all taken from a different range within the same sheet. What I need to do is to make it so that whenever a user chooses an item from one of the lists (in no particular order) the same item has to disappear from all the other lists in the range A1:B4. In other words, if a user selects "1" in A3, the "1" will not appear in any other list in the above-mentioned range. If he will then choose "2" in "A2", both "1" and "2" won't be available to pick in the remaining lists. I'm absolutely sure it's doable (hell, everything is), but I can't figure out how exactly. Please help me figure this one out, if you can. Thanks!

[wfm007]

There u go