The hypolipidemic response of rice bran oil (RBO) was investigated in nonhuman primates fed semi-purified diets containing blends of oils which included RBO at 0-35% Kcals as dietary fat. The studies demonstrated the following: (a) the degree of reduction of serum total cholesterol (TC) and low density lipoprotein cholesterol (LDLC) was highly correlated with initial serum cholesterol levels of the monkey on the stabilization diet; (b) the content of rice bran oil in the diet was the predominant factor influencing serum TC, LDLC and apoB causing up to a 40% reduction in LDLC without affecting high density lipoprotein cholesterol (HDLC) when RBO was the sole dietary oil fed; (c) the cholesterol-lowering capabilities of RBO were not explained by its fatty acid composition. These studies suggest that RBO may be an additional vegetable oil which lowers serum cholesterol levels by unique mechanisms which will require further study.