A major impediment to progress in research in onchocerciasis has been the lack of a suitable animal model. However, chimpanzees can be reliably infected by injection of living third-stage larvae of Onchocerca volvulus, and develop a pattern of infection that closely resembles that seen in humans. This includes the formation of nodules by adult worms, the subcutaneous distribution of microfilariae and the apparent lack of development of resistance to infection after repeated larval challenge. Ocular lesions resembling those in humans have not been observed in animals studied so far, but this may reflect in part the low intensity of infection and the limited time of observation. We infected 18 chimpanzees by subcutaneous injection of 250 third-stage larvae of O. volvulus. Six received ivermectin on day 1, another six received ivermectin on day 28 after infection, and six received no drug. Four control animals received no infective larvae and no drug. During the first year after infection, all three infected groups, but not control animals, have shown a clear increase in their lymphocyte blastogenic response to crude O. volvulus antigen in vitro. Antibody responses have also been increasing with time. Further assessment of the cellular and antibody responses in the infected chimpanzees is under way, as is evaluation of the effects of ivermectin on the course of infection.