The immunochemical potency of the blood group A and B specific urinary difucosyl pentasaccharides was compared with that of mono- and difucosyl type 2 reduced oligosaccharides of soluble blood group A and B substances. The urinary difucosyl oligosaccharides were as active as the corresponding difucosyl compounds from blood group substance but were less effective inhibitors than the reduced monofucosyl blood group oligosaccharides in most sera and lectin systems used. With one A anti-A system, however, the urinary A difucosyl pentasaccharide was more active. The difucosyl B oligosaccharides gave slight inhibition in a Leb anti-Leb system whereas the difucosyl A pentasaccharide from urine was inactive.