Phil,
yes, the reason is exactly b/c you already have a Worksheet_Change
macro in the sheet. You will need to incorporate both functionalities
under the same name. I.e., you will have to insert just the BODY of my
macro (i.e. w/o the first and last line End Sub), in the existing
macro.

For this to be done properly you need to specify all the checks that
you need to perform. Which ones should disallow any further action,
which ones should just produce a warning and in what order you want to
make the various checks. Write back and we will somehow combine the two
(is there any more?)

Kostis