On 2005-05-26 17:54:18 +0100, "Bernie Deitrick" <deitbe @ consumer dot
org said:

Martin,

You can use Data Validation to prevent multiple entries.

Select the three columns, all rows, where the values might be entered.
Let's say columns A, B and C, starting in row 2. A2 will be the activecell.

Use Data | Validation...., select "Custom" under "Allow", and in the
"Formula" area, enter

=COUNTA($A2:$C2)=1

You can modify the message to reflect your constraints.

HTH,
Bernie
MS Excel MVP

That works great.

Thank you very much.