A trisaccharide, which is a major carbohydrate component of the milk of the tammar wallaby and the grey kangaroo, has been identified by chemical, enzymic, g.l.c.-m.s., and n.m.r. methods as O-beta-D-galactopyranosyl-(1 yields 3)-O-beta-D-galactopyranosyl-(1 yields 4)-D-glucose (3'-galactosyl-lactose).