A nonhuman primate species infected with Schistosoma haematobium provided a model system for controlled studies on bilharzial bladder cancer. Urinary excretion of tryptophan metabolites by capuchin monkeys (Cebus apella) was similar to that of humans when expressed per g creatinine. Liver tryptophan oxygenase activity of the capuchin monkeys was comparable to that of humans. Excretion of 3-hydroxykyn-urenine and 3-hydroxyanthranilic acid was elevated above control levels in capuchin monkeys infected experimentally with S. haematobium. The capuchin-S. haematobium system closely resembles the human bilharziasis system and offers a reproducible laboratory model system for the controlled study of the parasitology, pathogenesis, and biochemistry of bilharzial bladder cancer.