4-Methylumbelliferyl N-acetyllactosaminide and 4-methylumbelliferyl sialyl N-acetyllactosaminides, which are used for the assay of sialytransferase, neuraminidase and fucosyltransferase, were synthesized, respectively, by the beta-D-galactosidase from Bacillus circulans and by a recombinant rat alpha 2,3-(N)-sialyltransferase or rat liver alpha 2,6-(N)-sialyltransferase with CMP-N-acetylneuraminic acid as donor.