The incidence of spontaneously occurring benign hepatomas in CSH male mice is strikingly lower among animals fed a semi-purified ration

benign hepatomas in CSH male mice is strikingly lower among animals fed a semi-purified ration containing 9 per cent casein than among those fed a diet containing 18 per cent casein as the principal source of protein (11). From the viewpoint of nutrition, the main differences between the diet containing 9 per cent and that with 18 per cent casein are (a) the total amount of protein and, more particularly, (b) the amount of specific essen tial amino acids, chiefly methionine and cystine ; in a diet with 9 per cent casein these sulfur-contain ing amino acids are not present in amounts optimal for the growth of mice or the efficient utilization of the ingested ration. The following cx periments were performed to determine whether either of these factors was responsible for the ob served difference in hepatoma formation.