2571,

You can make a custom rule within Data Validation which could prevent entry in 2 cells if a 3rd cell is not blank. If eg you're dealing with cells A1, B1 & C1, the following:

=AND(B1="",C1="")
as a custom DV rule for A1 will prevent any entry in A1 unless both B1 & C1 are empty. You can change the cell references as necessary to prevent entry in cells B1, then C1, unless both other cells are empty.

Hope this helps.



EDIT: sorry SixthSense, didn't see your (undoubtedly cleaner!) response while I was typing.