Hi
one way: use data validation:
- select cell B1
- enter the validation formula: =A1=""

select cell A1 and enter the validation formula: =B1=""

Frank


I need to lock cell B1, for instance, when A1 contains data, and
vice-versa. I only want a user to have the ability to populate one of
the two fields so that I can write a simple sum formula.

Thanks in advance for the help.