Male rats were fed for 4 weeks cholesterol-free diets containing proteins from different sources, milk, fish, egg yolk, soybean, rice and peanut. The antihypercholesterolemic effect of vegetable proteins compared to animal proteins was certified. There was a negative correlation (γ = -0.74) between the serum cholesterol level and the arginine/lysine ratio of dietary protein, suggesting a role of this ratio in determining the serum cholesterol level. In general, the fasting level of circulating insulin was lower whereas that of glucagon was higher on feeding vegetable proteins, thus resulting in a fall of the insulin/glucagon ratio. Changes in the hormonal status may be relevant to the protein effect