Bovine milk was found to contain, in soluble form, an enzyme which transfers galactose from UDPgalactose to glucosylceramide. This enzyme was partially purifed by the same procedure used to isolate the galactosyltransferase of lactose synthetase. The partially purified enzyme required detergents for activity, had a pH optimum of 7.2–7.3 and required Mn 2+. The apparent K m calculated for glucosylceramide was 1.33 · 10 −4 M. With glucosylceramide as acceptor the product of the reaction was identified as lactosylceramide by autoradiography on thin-layer chromatograms. Lactosylceramide was also an effective acceptor for the transferase reaction but neutral glycosphingolipids or gangliosides with terminal galactose or N-acetylgalactosamine residues were ineffective or poorly effective as acceptors. Addition of α-lactalbumin inhibited the transferase reaction.