Male growing rats were fed diets containing either raw field bean (Vicia faba L.) or casein as the only source of protein. Diets were fed ad libitum for 5 weeks. Body weight gain was significantly impaired, and rates of both in vivo and in vitro intestinal absorption of D-glucose, D-galactose and L-leucine were reduced, in rats fed the legume diet. The addition of methionine to the bean diet did not alter either growth or intestinal absorption rates. No significant differences in intestinal oxygen uptake were found between the treatment groups. It is suggested that the antinutritive substances contained in the raw legume Vicia faba are responsible for the inhibitory effects observed in this experiment.