Twenty-seven 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 non-human primates for their potential carcinogenicity and other long-term toxic effects. Sixteen of these substances have not as yet demonstrated carcinogenic activity, although some have been on test for less than 3 years. Eight of the compounds are carcinogenic in non-human 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 leukemias. 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 cases of malignant tumors have been diagnosed in animals treated with adriamycin, butter yellow, sterigmatocystin, and N-methyl-N'-nitro-N-nitrosoguanidine.