Recent progress in molecular glycobiology has revealed the important biological roles of numerous glycoconjugates [ 11. In particular cell-surface carbohydrates play key roles in cellular communication processes via selective adhesion phenomena [2, 31. They guide e.g. leukocyte extravasation [4], the homing of lymphocytes and adhesion of myeloid cells to activated platelets and the endothelium [5][6][7]. Besides these interactions carbohydrates are involved in fertilization events, cell growth, and parasitic infections of viruses and bacteria 181. A very recent finding is the involvement of certain cell surface a-galactosides in hyperacute rejection reactions; this is of pharmaceutical interest in organ transplantation 191.This makes sugars worthwhile targets for pharmaceutical applications [ 10, 1 11. Although chemical synthesis of oligosaccharides has reached a mature stage [ 12-141 the overall synthetic sequences are still lengthy and cumbersome [ 151. Biologically active carbohydrates are, moreover, composed of various different monosaccharide units and are often only active if presented in a polyvalent or multiantennary array to their respective receptors [ 16, 171. These multiantennary heteropolymers, in particular, have resisted efficient chemical synthesis. Recently biocatalysts have been found to be valuable tools for complementing the classical synthetic approach [ 181.Nature has invented several ways of assembling complex oligosaccharides [ 11. The Leloir pathway is one of the preferred routes of mammalian systems. The Leloir enzymes-the glycosyl transferases-transfer a monosaccharide unit from a nucleotide-activated donor substrate to a growing oligosaccharide chain with rigorous regio-and stereoselectivity [19] (see Scheme 1). Exclusively one of the many OH groups of the acceptor is glycosylated, in either an a or p mode! Neither the donor nor the acceptor sugar needs protection from the highly homofunctional reactants-often OH groups only are present on the sugars. Thus, selective protecting group manipulations, a preponderant task in the classical, chemical oligosaccharide assemblage can be reduced to an absolute minimum.