The structure of carbohydrate chains of dog gastric mucus glycoprotein bearing Forssman antigenic determinant was investigated. The reduced oligosaccharides obtained by alkaline borohydride reductive cleavage of the glycoprotein were chromatographed on a immunoaffinity column containing monoclonal anti-Forssman antibody coupled to cyanogen bromide-activated Sepharose 4B. The bound oligosaccharides, eluted from the affinity column with 6 M urea, separated on thin layer chromatography into two major components. These two compounds, representing 92% of the absorbed oligosaccharides, were purified to homogeneity, and their structures and antigenic properties were determined. Based on the results of immunological assays, analysis of sugar composition, sequential degradation with specific glycosidases, and methylation analyses of the intact and enzyme-degraded compounds, we propose the following structures for these oligosaccharides. (formula, see text).