O-β-D-Galactopyranosyl-(1→ 4)-O-β-D-galactopyranosyl-(1→4)-D-glucopyranose (designated as 4'GL) is produced from lactose by Cryptococcus laurentii. The influence of chronic ingestion of 4'GL on body weight gain, organ weight, serum lipids, and liver lipids was investigated in rats. The body weight gains of the 5 % and 10% 4'GL-diet groups were higher than that of the control group. Food intake and fecal dry weight were significantly increased (p<0.05) by 4'GL feeding. The 4'GL diet produced a significant increase (p<0.01) in the wet weight and contents of both the cecum and the colon. However, no significant increase was observed in the weight of the stomach, small intestine, liver, or other organs. The effects of 4'GL on serum and liver lipid levels were not observed in this experiment. The digestion of 4'GL was measured in vitro using the artificial gastric juice, α-amylase of human saliva, α-amylase of hog pancreas, and mucosa of rat intestine. 4'GL was not hydrolyzed by these enzymes. Long-term ingestion of 4'GL did not cause any induction of 4'GL hydrolyzing enzyme activity in the rat small intestine.