Dietary protein has been implicated in atherosclerosis since the earliest studies of diet in relation to cardiovascular disease. The increase in serum cholesterol in rabbits following the supplementation of lysine shows that the amino acid content of the diet is an important factor in altering serum cholesterol levels. A lysine/arginine ratio of 0.3 for almond-meal feeding results in serum cholesterol levels in rabbits that are no lower than can be achieved by our previously fed plant-protein laboratory chow. These apparent anomalies suggest that other amino acids are also important in modulating serum cholesterol, besides the lysine content of the diet. The effects of dietary proteins on serum cholesterol levels is determined by the amino acid composition of proteins in the diet, which indicates a causal relationship between dietary proteins and blood cholesterol levels. Data from both animals and humans show that several amino acids, besides lysine and arginine, are involved in the control of blood cholesterol levels.