but I would like to add a further condition

Not possible with the Data Validation feature. You can have a list, as now,
or have a test on cell A2 (using Custom instead of List) but not both.

One kludgy compromise that comes to mind... The cells that comprise your
list for validation purposes could be formulas that return "" if the key
cell is blank. Like:

=IF($A$2="","","a")
=IF($A$2="","","b")

So then no matter what is entered it fails the validation.

Your error message could say that either key cell has no entry or a valid
list item wasn't entered.

--
Jim
"Pastel Hughes" wrote in message
...
|I have created a form which has several cells (eg C3, C4, C5)
| that have all LIST validation, where the "error alert" is set to
| "information"
| in order to allow entries that are not in the lists.
| These validations work fine; but I would like to add a further condition
| to these cells that would inhibit input,
| in the event that another specific "key" cell (eg. A2) is blank,
| as well as producing a message informing the user what remedial action to
| take.
| I'm afraid that I am at a bit of a loss as to how to proceed.