This report covers a 32-year period of an ongoing chemical carcinogenesis study in nonhuman primates, which was initiated by the National Cancer Institute in 1961. Autopsy records of 373 breeders and normal controls showed very low incidence of spontaneous malignant tumors in cynomolgus (1.5%) and rhesus (2.8%) monkeys, but considerably higher incidence in African green monkeys (8%). A large number of substances including a variety of food additives, food components, environmental contaminants, N-nitroso compounds, "classical" rodent carcinogens, antineoplastic agents, and immunosuppressive agents have been evaluated for long-term carcinogenic activity. Food components tested which are probably most relevant to human exposure are the artificial sweeteners, cyclamate and saccharin. After 22 years of continuous dosing, neither cyclamate nor saccharin have shown any evidence of carcinogenic effects. Similarly, the tumorigenic potential of arsenic and DDT was negligible after dosing for 15-22 years. In contrast, the fungal food contaminants, aflatoxin B1 (AFB1) and sterigmatocystin (SMT), were found to be potent hepatocarcinogens. AFB1 also induced adenocarcinomas of the pancreas, osteosarcomas, and other tumors. Also, the aglycone of cycasin, MAM acetate, induced a variety of tumors, but primarily hepatocellular and renal cell carcinomas. The compounds most recently introduced into the colony include three heterocyclic amines present in cooked meat. One of these compounds, 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) has proven to be one of the most potent hepatocarcinogens in the history of the monkey project, inducing malignant liver tumors in 65% of animals over a 7-year period of exposure. Of the classical rodent carcinogens studied, urethane was the only one which produced malignant tumors in the monkeys. Conversely, all except two of the N-nitroso compounds were carcinogenic. Diethylnitrosamine (DENA) was the most potent and predictable hepatocarcinogen in cynomolgus, rhesus, and African green monkeys. However, when administered intraperitoneally to galagos (a prosimian), DENA induced primarily mucoepidermoid carcinoma of the nasal cavity. N-Methyl-N-nitrosourea (MNU) was the only carcinogen persistently producing tumors in the digestive tract, mostly squamous cell carcinomas of the esophagus. Among the antineoplastic and immunosuppressive agents, procarbazine (MIH) was the only unequivocal carcinogen, with a 33% tumor incidence, causing acute nonlymphocytic leukemia in most of the cases.