Growth rates of rats fed lyophilized diets of yogurt, milk, 100% lactose-hydrolyzed milk, and 100% lactose-hydrolyzed yogurt were compared. No differences were significant in weight gains between the animals fed lactose-hydrolyzed diets over 3 wk. Further more, rats fed the lactose-hydrolyzed diets had significantly larger weight gains during the 1st wk than those fed unhydrolyzed diets. This may have been from the deleterious effect of lactose, which was in concentrations of 43% in milk and 36% in yogurt. However, by the 2nd wk, rats fed yogurt had growth rates similar to those fed lactose-hydrolyzed diets, and by the 3rd wk, no differences of growth rates were significant. Continued consumption of high concentrations of lactose improved digestion of lactose. Yogurt fermentation produced growth benefit as compared to the milk from which it was made by reducing the lactose content and by supplying microbial lactase activity.