Let's say you want to do data validation in cells A1:A100.

Select those cells from A1 to A100.

On the data validation entry, select Custom and enter

=countif(A1:A$1,A1)=1

Enter the appropriate error alert.

If it were me, I'd probably change the color of the cells that are being
validated so that I'd know it was there.





"Satraj" wrote in message
...
Say if I had the folloing data in column A1:A3 and I try to enter 300 in
A4
again, using data validation I want to be able to be stopped from entering
that value again.
Column A
200
300
400