A glycosidase from Aspergillus oryzae catalysed the stereospecific formation of various derivatives of the Tn antigen, D-GalNAc alpha 1-->O-L-Ser, employing D-GalNAc alpha-OPhNO2-p as glycosyl donor and different N- and C-protected L-Ser derivatives as acceptors. The same glycosidase preparation was also useful for stereospecific preparation of D-GalNAc alpha 1-->O-L-Thr and D-GlcNAc beta 1-->O-L-Ser derivatives. Yields were in the range 10-50% depending on the type of acceptor. Lipase from porcine pancreas was used for specific hydrolysis, generating a Tn antigen derivative with a free carboxyl group. This facilitates the use of the derivatives in e. g. solid phase synthesis of glycopeptides.