The scope of glycosyltransferases in the synthesis of oligosaccharide analogues has been expanded to include the use of "retaining" enzymes acting on complex tertiary C-branched disaccharide accepters. The enzymes studied were alpha 1,3-galactosyltransferase (alpha 1,3-GalT, EC 2.4.1.151), which uses Gal beta 1-->4Glc beta-OR as the acceptor, and recombinant blood group A [alpha 1,3-GalNAc transferase (GTA), EC 2.4.1.40] and B [alpha 1,3-Gal transferase (GTB), EC 2.4.1.37] glycosyltransferases, which use Fuc alpha 1-->2Gal beta-OR as the acceptor in their biosynthetic reactions. Chemical synthesis produced acceptor analogues in which the ring carbon bearing the OH groups undergoing enzymatic glycosylation in the natural reactions had their C-H bond replaced with a C-methyl or C-propyl bond. The tertiary C-methyl alcohols proved to be kinetically competent substrates which permitted the enzymatic preparation of the expected product trisaccharides in near quantitative yields.