Trying to use conditional format when a cell is also in a separate worksheet range

Hi,
Column A of Sheet 1 has the results of an audit of serial #'s perform.
Column A of Sheet 2 lists only the serial #'s I'm interested in. (A subset of the column mentioned prior)

How do I use a formula in conditional formatting to highlight the cells in Sheet 1 that are present in sheet 2?

You can use CF with formula ("Use a formula to determine which cells to format"). In sheet 1 try,

=COUNTIF(Sheet2!$A:$A,$A1)

where A1 is the starting cell for the conditional formatting. IF countif is >=1, i.e. if value is also present in Col A in Sheet1, the formula will recognize it as True.

Thanks for the suggestion. I got the error "You may not use reference operators or array constants for conditional formatting criteria when I tried it