A spectrophotometric method that can monitor the trans-sialidase catalyzed reaction was devised by using a chromogenic substrate, o-nitrophenyl-beta-d-galactopyranoside (ONPG), as a sialic acid acceptor. This method yielded results that were consistent with those from HPLC, and could be very conveniently used in the screening of sialic acid donor or inhibitor of trans-sialidase.