Groups of five 3-kg Kenyan monkeys, Cercopithecus aethiops, were exposed individually to 150,600 or 1500 Schistosoma mansoni cercariae per monkey. Three monkeys died soon after the infections became patent and the survivors were autopsied 4 months after exposure. Mortality and most haematological, parasitological and pathological sequelae of infection were dose-related, but not the white cell response or changes in the levels of serum proteins or fibrinogen. No gross liver fibrosis was seen. Comparison of this study with earlier ones on related cercopithecine monkeys suggests that the vervet closely resembles the baboon in its response to S. mansoni infections. Difficulties in managing and maintaining vervets can be overcome by using colony-bred or properly adapted feral animals. Thus, the vervet provides a cheaper, more readily available primate model for experimental S. mansoni studies. A prolonged infection, sufficiently heavy to permit reliable parasitological monitoring without undue mortality, should be provided by 150 S. mansoni cercariae per kg body-weight, using the Kenyan strains of vervet and parasite.