A novel chemoenzymatic approach to synthesize neoglycoproteins containing high-mannose-type oligosaccharides is described. p-Isothiocyanatophenyl-beta -D-glucopyranoside (Glc-ITC) was transferred to the reducing end of the high-mannose-type oligosaccharides using a transglycosylation activity of endo-beta -N-acetylglucosaminidase A (Endo-A), A novel oligosaccharide, Man(6)GlcNAc-Glc-ITC, was synthesized as a coupling reagent for lysyl and N-terminal residues of the protein moiety. The neoglycoconjugate was coupled with several nonglycosylated proteins such as ribonuclease A, lysozyme, and alpha -lactalbumin. Between one and four high-mannose-type oligosaccharides were incorporated per molecule of these proteins. This method should be very useful for the synthesis of neoglycoproteins with homogeneous high-mannose-type oligosaccharides.