Effects of low-fat (1% corn oil) diets containing an isonitrogenous level (3.2%) of either casein, soybean protein isolate or three soybean globulin fractions (total, 11S and 7S globulins) on plasma and liver lipids and on the excretion of fecal neutral sterols were examined in rats. When diets free of cholesterol were fed, the concentration of plasma cholesterol was significantly lower in all groups of rats given different soybean proteins than in the animals given casein. 11S globulin tended to produce a lower level of plasma cholesterol as compared with the 7S counterpart. Although the net decrease due to feeding plant proteins in high density lipoprotein cholesterol was to some extent greater than that of very low and low density lipoprotein cholesterol, the percentage of the former to total cholesterol remained in the normal range. The concentration of liver cholesterol appeared to be lower in rats given soybean proteins. Rats fed soybean globulins, as compared with the animals fed casein, tended to excrete larger amounts of neutral sterol into the feces. These soybean-effects disappeared when cholesterol and cholic acid were added to diets. The results indicated that the antihypercholesterolemic effect of soybean protein was attributable to protein per se, and not to the carbohydrate components which were associated with the protein preparation.