Abstract: A method is disclosed for modifying the carbohydrate moiety on glycoproteins to facilitate the structural and functional analysis of said glycoproteins such as by NMR spectroscopic analysis and crystallography which comprises treating glycoprotein-secreting mammalian cells having low endomannosidase activity under cell culture maintenance conditions with a glucosidase I inhibitor, and after secretion and purification, subsequent treatment of the active glycoprotein with endoglycosidase H to thereby provide a glycoprotein with a single GlcNAc residue at each glycosylation sequon. The preferred mammalian cells are CHO cells and the preferred glucosidase I inhibitor is N-butyl deoxynojirimycin.