The carcinogenic effects of 26 substances, including antineoplastic and immunosuppressive agents, contaminants of human foodstuffs, rodent carcinogens, pesticides, and artificial sweeteners are being evaluated in four species of non-human primates. Nineteen of these substances have not as yet demonstrated carcinogenic activity, although some have been on test for less than two years. Seven of the test compounds are carcinogenic in non-human primates, producing tumors in 13-100% of the treated animals. The effects of five of the seven compounds (DENA, DPNA, 1-nitrosopiperidine, aflatoxin B1 and MAM-acetate) were manifested primarily as hepatocarcinogenicity. In addition, 1-methyl-1-nitrosourea induced squamous cell carcinomas of the oropharynx and esophagus, with the esophageal tumors in monkeys possessing many clinical and morphologic similarities to human esophageal carcinoma. Long-term treatment with procarbazine resulted in an increased incidence of malignancies, approximately one-half of which were acute myelogenous leukemia. Alpha-fetoprotein has been found to be a useful marker for diagnosing and for following the success of treatment of hepatocellular carcinoma in non-human primates. Studies on chemo-prevention of cancer in non-human primates using various nucleophiles are in progress.