The enzyme-catalyzed hydration2 of D-galactal and the enzyme-catalyzed hydrolysis of fl-D-galactopyranosides are similar as far as the products are concerned. In the case of hydration of D-galactal, a covalent, 2deoxy-D-Zyxo-hexosy1("2deoxy-D-galactosyl")enzyme intermediate could be isolated' . As shown by kinetic studies3 and initial-burst experiments4, hydrolysis of oNPGal *** also proceeds through a (not necessarily covalent) D-galactosyl-enzyme that seems to be present in appreciable proportion. These similarities led us to assume the same type of intermediate in both reactions, and to try affinity labelling by denaturation using oNPGal, which is one of the best-investigated substrates' for p-D-galactosidase. Affinity labelling by quenching had previously been conducted with sucrose phosphorylase from Pseudomonas saccharophila"", levansucrase (EC 2.4.1 .lO) from Bacillus subtili?, and O-D-glucosidase (EC 3.2.1.21) from Aspergillus wentii'. D-[6-%I] Galactose was prepared from I,2 :3,4di-U-isopropylidene-cu-D-gaZactohexodialdo-I ,5-pyranoser' (37 pmol) by reduction with NaBsHe (9.26 nmol, 54 Ci/mmol)