Human plasma galactoglycoprotein (Mr = 81,000) which was recently identified and characterized (Schmid, K., Mao, S. K. Y., Kimura, A., Hayashi, S., and Binette , J. P. (1980) J. Biol. Chem. 255, 3221-3226) was found to possess an unusually high carbohydrate content (76%) comprised of NeuAc, Gal, Man, Fuc, GalNAc, and GlcNAc. The aim of the present investigation was to elucidate the primary structure of the oligosaccharide units of this protein. For the study of the O-glycosidic oligosaccharide chains, the protein was subjected to beta-elimination and the resulting oligosaccharide preparations were analyzed by 500-MHz 1H NMR spectroscopy. The structure of the predominant glycan, a hexasaccharide: (formula; see text) and that of a tetrasaccharide: NeuAc alpha(2----3)Gal beta(1----3)[NeuAc alpha (2----6)]GalNAc-ol, were determined. The protein possesses approximately 40 hexasaccharides and 3 tetrasaccharides/mol. For the isolation of its N-glycosidic oligosaccharide chains, the protein was exhaustively digested with proteases followed by chromatography of the desialyzed resulting glycopeptide fraction on concanavalin A-Sepharose. 500-MHz 1H NMR spectroscopy of the obtained preparations revealed the presence of 3 diantennary N-glycosidic chains that are extended with a Fuc residue at the Asn-linked GlcNAc.