Twenty-eight substances, including antitumor agents, contaminants of human food-stuffs, rodent carcinogens, pesticides, and artificial sweeteners have been or are being evaluated in four species of nonhuman primates for their potential carcinogenicity and other long-term toxic effects. Seventeen of these substances have not as yet demonstrated carcinogenic activity, although some have been on test for less than 4 years. Eight of the compounds are carcinogenic in nonhuman primates, producing tumors in 19-100% of the treated animals. 1-Methyl-1-nitrosourea induced squamous cell carcinomas of the oropharynx and esophagus, with the esophageal tumors possessing clinical and morphologic similarities to human esophageal carcinoma. Long-term treatment with procarbazine resulted in an increased incidence of malignancies, one-half of which were acute leukemia. The effects of 6 of the 8 compounds (DENA, DPNA, 1-nitrosopiperidine, aflatoxin B-1, MAM-acetate and urethane) were manifested primarily as hepatocarcinogenicity. Single cased of malignant tumors have been diagnosed in animals treated with adriamycin, butter yellow and sterigmatocystin.