It is clear that our knowledge regarding the effects of low-protein and high-protein diets for each species is influenced by the number of studies and experimental conditions. It is therefore quite impossible to attempt to suggest an accurate general conclusion. Moreover, as the effects of low-and high-protein diets differ according to species, any extrapolation of the data to human use must take into consideration the species investigated. When the level of proteins in the diet decreases or increases, the serum cholesterol concentrations are enhanced in birds, rabbits, and rodents (rats, mice, hamsters). Chickens, however, react differently from other species. In calves and monkeys, reactions vary according to subspecies. In pigs, serum cholesterol would appear to be particularly sensitive to varying levels of dietary proteins. In dogs, too few experiments have been conducted for any conclusion to be proposed. Of all the proteins investigated, casein produces the greatest increase in serum cholesterol concentrations. In monkeys, a protein intake in conformity with protein requirements can maintain normal lipid, lipoprotein, and cholesterol serum levels. A low-protein diet increases VLDL and HDL particle levels, whereas a high-protein diet increases serum cholesterol, lipid, and lipoprotein concentrations.