Conditional formatting will do it with a formula of say

=COUNTIF(A:IV,A1)1

--
HTH

Bob Phillips

"css" wrote in message
...
If I enter a number into any cell which has already been entered into
another
cell, regardless of row or column, is there any way in which my attention
could be drawn to the fact that this number is already in the worksheet,
i.e.
by highlighting or something similar.