A new approach for the highly specific preparation of L-serine conjugates of lactosamine and Ga1 beta 1-3GalNAc is described. Thus, the L serine derivative of lactosamine Ga1 beta 1-4GlcNAc beta-O-(N-Z)-Ser-OEt, was obtained from lactose, employing GlcNAc beta-O-(N-Z)-Ser-OEt as acceptor and a yeast beta-galactosidase as catalyst. Ga1 beta 1-3GalNAc alpha-O-(N-Alloc)-Ser-OMe was obtained from lactose, employing GalNAc alpha-O-(N-Alloc)-Ser-OMe as acceptor and beta-galactosidase from bovine testes as catalyst.