The insulin receptor, an integral membrane glycoprotein, is synthesized as a single-chain precursor that is cleaved to produce two mature subunits, both of which contain N-linked oligosaccharide chains and covalently linked fatty acids. We report that the β-subunit also contains O-linked oligosaccharides. The proreceptor, α-subunit, and β-subunit were labeled with [3H]mannose and [3H]galactose in the presence or absence of an inhibitor of O-linked glycosylation. Tryptic peptides from each component were separated by reverse-phase high-performance liquid chromatography. N- and O-linked oligosaccharide chains were identified on these peptides by specific enzymatic digestions. The proreceptor and α-subunit contained only N-linked oligosaccharides, whereas the β-subunit contained both N- and O-linked oligosaccharides. The O-linked oligosaccharide chains were attached to a single tryptic fraction of the β-subunit, which also contained N-linked chains. This fraction was further localized to the NH2-terminal tryptic peptide of the β-subunit by specific immunoprecipitation with an anti-peptide antibody with specificity for this region. Binding of insulin and autophosphorylation of the β-subunit were not dependent on O-linked glycosylation, because cells grown in the presence of the inhibitor exhibited a normal dose response to insulin. Therefore, the insulin receptor contains O-linked oligosaccharides on the NH2-terminal tryptic peptide of the β-subunit, and these O-linked oligosaccharides are not necessary to the binding or autophosphorylation function of the receptor.