The epidemiological studies suggest that aflatoxins, the toxic metabolites of the ubiquitous mold Aspergillus flavus, may play a significant role in the evolution of hepatocellular carcinoma in man in certain geographic areas of the world. To ascertain their carcinogenicity in nonhuman primates, we have administered highly purified aflatoxin B1, intermittently in the diet at 2 ppm, to 10 female and 8 male tree shrews. The tree shrew (Tupaia glis) is a nonhuman primate occurring throughout Southeast Asia which can be reared easily in captivity. Of 12 animals that survived, 6 of 6 female (100%) and 3 of 6 male (50%) tree shrews developed hepatocellular carcinomas between 74 and 172 weeks after the beginning of the experiment. None of the 8 control animals developed liver cancers. The estimated total amount of aflatoxin B1 consumed by these animals ranged from 24 to 66 mg. The development of liver tumors did not follow a specific pattern; considerable variation in hepatocellular responses to aflatoxin B1 was noted in these animals. In 2 tree shrews, the liver tumors were associated with severe post necrotic scarring; in the other 7 tumor-bearing livers, only mild to moderate portal fibrosis was encountered. This individual variation in hepatocellular response and in the amount of aflatoxin B1 required to induce hepatocellular carcinomas is attributed to inherent differences in the susceptibility within a given species of outbred animals and suggests extreme caution in proposing the "permissible" or "safe" levels of contamination of carcinogens in the food-stuffs.