N-Acetyl-lactosamine(beta-D-Gal p-(1-->4)-D-Glc pNAc) was synthesized regioselectively with the aid of the transglycosylation activity of beta-galactosidase isolated from Diplococcus pneumoniae using p-nitrophenyl beta-D-galactopyranoside as the donor. Also, transglycosylation of the sialyl group in an alpha-(2-->8)-linked sialic acid dimer or p-nitrophenyl glycoside of sialic acid to N-acetyl-lactosamine was performed using sialidases of various origins. When sialidase from Clostridium perfringens, Arthrobacter ureafaciens, or Vibrio cholerae was used, alpha-(2-->6)-linked sialyl N-acetyl-lactosamine was obtained regioselectively. In contrast, when sialidase from newcastle disease virus was used, the alpha-(2-->3)-linked isomer was obtained regioselectively. The regioselectivity of the transglycosylation reaction using beta-galactosidase and sialidase was compared with hydrolysis specificity toward the same linkages.