You can add an Error Alert Message, in the data validation dialog box.

Also, I've revised my validation setup (changes are marked '>>):

Create a named range with your list of options, e.g. MyList.
There are instructions here:
http://www.contextures.com/xlNames01.html

'>> In cell G1, type: =""

Choose Insert>Name>Define
Type the name: NoList
'>> In the Refers to box, type: =Sheet1!$G$1
You can change the Sheet1!$G$1 reference to a sheet and cell
in your workbook
Click OK

Select cells A1:B1
Choose Data>Validation
From the Allow dropdown, select List
In the Source box, type:
=IF(COUNTA($A$1:$B$1)=0,MyList,NoList)
'>> Remove the check mark from 'Ignore Blanks'
Click OK

Keith wrote:
> Debra,
> Thanks this works fine. Is there a way to have an Error Alert generated when
> counta =1 (when the one of the cells has data selected from the list)?
>
> Thanks,
> Keith
>
>
>
> "Debra Dalgleish" <dsd@contexturesXSPAM.com> wrote in message
> news:42725588.3000400@contexturesXSPAM.com...
>
>>Create a named range with your list of options, e.g. MyList. There are
>>instructions here:
>>
>> http://www.contextures.com/xlNames01.html
>>
>>Choose Insert>Name>Define
>>Type the name: NoList
>>In the Refers to box, type:
>>
>> =OFFSET(Sheet1!$G$1,0,0,0,1)
>> You can change the Sheet1!$G$1 reference to a sheet and cell
>> in your workbook
>>
>>Click OK
>>
>>Select cells A1:B1
>>Choose Data>Validation
>>From the Allow dropdown, select List
>>In the Source box, type:
>>
>> =IF(COUNTA($A$1:$B$1)=0,MyList,NoList)
>>
>>Click OK
>>
>>Keith wrote:
>>
>>>Two cells, using data validation | custom... COUNTA($A1:$B1)<2
>>>This works fine to prevent data in both cells.
>>>But I also want to use a drop down list for the data selection in A1 and
>>>B1,
>>>How can I do both?
>>>Thanks,
>>>Keith
>>>
>>>
>>>
>>
>>
>>--
>>Debra Dalgleish
>>Excel FAQ, Tips & Book List
>>http://www.contextures.com/tiptech.html
>>
>
>
>


--
Debra Dalgleish
Excel FAQ, Tips & Book List
http://www.contextures.com/tiptech.html