We have investigated some of the biochemical characteristics of the glycosylated forms of α2u-globulin, the major protein of adult male rat urine. Labeled α2u-globulin that was secreted into the culture medium of primary monolayer cultures of hepatocytes was immunoprecipitated from the medium with antibody specific for urinary α2u-globulin and the complex used for further study. α2u-Globulin existed in the medium as a high-molecular-weight aggregate that could be dissociated with reducing agents and detergents into two glycosylated molecules of Mr 75000 and 50000 and one nonglycosylated molecule of Mr 20000. The glycosylated forms of α2u-globulin were shown by kinetic studies not to be precursors of the molecule of Mr 20000. When the glycoproteins were synthesized in the presence of radioactive sugar and amino acid precursors, the ratio of sugar to amino acid suggested that the highest-molecular-weight glycoprotein had 1.5–2 times the carbohydrate content of the faster-migrating glycoprotein. The oligosaccharide moiety, isolated from an exhaustive pronase digest of the combined glycoprotein molecules, was of Mr 5000 as determined by gel nitration methods. Sequential glycosidase digestion with neuraminidase, β-galactosidase, and β-N-acetylglucosaminidase produced a core oligosaccharide of Mr 1200. Anion-exchange chromatography demonstrated that the charge microheterogeneity of the α2u-globulin glycopeptides could be abolished by treatment with neuraminidase. Our results indicate that the oligosaccharide moiety of α2u-globulin is similar to those oligosaccharides found on other serum proteins, and suggest that formation of the species of α2u-globulin of Mr 75000 and 50000 requires either the attachment of many oligosaccharide chains to a single polypeptide of Mr 20000 or the covalent linkage of poly-peptides containing one or more oligosaccharide chains to form dimers and trimers.