We have reported the isolation and characterization of a bovine cDNA clone containing the complete coding sequence for UDP-Gal:GalPI + 4GlcNAc a1 + 3-galactosyltransferase [Joziasse, D. H., Shaper, J. H., Van den Eijnden, D. H., Van Tunen, A. J. & Shaper, N. L. (1989) J . Biol. Chem. 264, 14290-142971. Insertion of this cDNA clone into the genome of Autographa californica nuclear polyhedrosis virus (AcNPV) and subsequent infection of Spodoptera ,frugiperda (Sf9) insect cells with recombinant virus, resulted in high-level expression of enzymatically active a1 + 3-galactosyltransferase. The expressed enzyme accounted for about 2% of the cellular protein; the corresponding specific enzyme activity was 1000-fold higher than observed in calf thymus, the tissue with the highest specific enzyme activity reported to date. The recombinant a1 + 3-galactosyltransferase could be readily detergent-solubilized and subsequently purified by affinity chromatography on UDP-hexanolamineSepharose. The recombinant a1 + 3-galactosyltransferase showed the expected preference for the acceptor substrate N-acetyllactosamine (GalP1 + 4GlcNAc), and demonstrated enzyme kinetics identical to those previously reported for affinity-purified calf thymus a1 Bid. Clzem. 260, 12927-129341. In pilot studies, the recombinant enzyme was examined for the ability to synthesize a1 + 3-galactosylated oligosaccharides, glycolipids and glycoproteins. By a combination of 'H-NMR, methylation analysis, HPLC, and exoglycosidase digestion it was established that, for each of the model compounds, the product of galactose transfer had the anticipated terminal structure, Galal + 3GalP1+ 4-R. Our results demonstrate that catalysis by recombinant a1 -+ 3-galactosyltransferase can be used to obtain preparative quantities of various a1 + 3-galactosylated glycoconjugates. Therefore, enzymatic synthesis using the recombinant enzyme is an effective alternative to the chemical synthesis of these biologically relevant compounds.The enzyme UDP-Gal:GalPl + 4GlcNAc a1 + 3-galactosyltransferase (a1 + 3-galactosyltransferase) is a Golgimembrane-bound enzyme that catalyzes the following reaction :Gala1 +3Gal/31+4ClcNAc-R + UDP The preferred acceptor structure, GalPl + 4GlcNAc, may be present at the non-reducing terminus of either glycolipids, glycoproteins or oligosaccharides (Blanken and Van den Eijnden 1985). Consequently, the a1 -+ 3-linked galactose occupies the terminal non-reducing position of N-acetyllactosamine-type carbohydrate chains and of lactosaminoCorrespondr.nce to