A blood group-characteristics obligosaccharide has been isolated from urine of human group O(H)-secretors. Its chemical composition and behaviour in paper chromatography strongly indicate that it is identical with lactodifucotetraose. Both substances have glucose in the reducing end group, and seem to give the same degradation products on partial acid hydrolysis. Lactodifucotetraose has previously been found only in pregnant and lactating women. © 1968.