Using 2003

The IsNumber() function gets a Compile error in an Excel VBA project. ("Sub
or function not defined")

I have used it previously in other VBA code. The procedure is not declared
Private nor is Option Private Module in effect.

I am calling the function from my Personal.xls for evaluating a cell in a
separate worksheet.

Which DLL library is it in?

What dumb obvious error did I do? Hit me.

Thanks Dennis