Six individual reduced pentasaccharides and hexasaccharides were isolated after sodium borohydride degradation of blood group substance H from pig stomach linings. Their structure was determined by means of periodate oxidation, methylation analysis, mass spectrometry, partial acid and enzymic hydrolysis and chromic anhydride oxidation. It has been found that all the oligosaccharides investigated contain the common tetrasaccharide core to which terminal alpha-L-fucose and N-acetyl-alpha-D-glucosamine residues are attached. Some problems of carbohydrate chain heterogeneity in blood group substances are discussed.