conditional formatting across worksheets

And between workbooks, but changing the ranges to suit your needs:

1. In the one workbook name the range containing the serial numbers, here I named it Data.  (highlight the range and choose Insert, Name, Define, type Data into the Names in workbook box and click OK).
2. Move to the other workbook and in an empty cell type = and then click on any cell in the other workbook to build a formula like
'[10-21-2008.xls]Sheet2'!$A5
3. Edit this formula so it reads
'[10-21-2008.xls]Sheet2'!Data
Press Enter and the formula will change to
='10-21-2008.xls'!Data
4. Highlight the formula on the Formula Bar and copy it, press Enter once to escape the Formula Bar
5. Choose the command Insert, Name, Define and enter a name like FSE1 in the Names in workbook box (if you are using 2007 make it four letter followed by a number to be on the safe side)
6. Click in the Refers to box, clear the existing entry and choose Paste.  This should paste the formula from step 3 into the Refers to box.  Click OK.
7. Select the range with your serial numbers say A1:A100 and choose Format, Conditional Formatting, Formula is from the first drop down, and in the second box enter =OR(A1=FSE1)  pick Format and choose what you want.  Click OK twice.