The heterocyclic aromatic amine 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) was evaluated for carcinogenic effects in macaques, primarily cynomolgus monkeys. IQ was administered by gavage five times a week at doses of 10 or 20 mg/kg. IQ induced hepatocellular carcinoma in 55% of the animals at the low dose and in 95% of the animals at 20 mg/kg. The average latent period at the high dose level was 43 months and that at the low dose was 60 months. Generally, the tumor nodules exhibited a well- to moderately well-differentiated hepatocellular carcinoma, and a trabecular pattern was most frequently seen. Pulmonary metastases were also found in several of the monkeys. Thus, IQ is a potent carcinogen in nonhuman primates and is a potential carcinogen for humans.