Two young male common marmosets (Callithrix jacchus) were injected intracerebrally and intraperitoneally with a crude brain homogenate prepared from a cow with bovine spongiform encephalopathy (BSE). Two other marmosets were similarly injected with brain homogenate from a sheep with natural scrapie. The two animals injected with scrapie material developed neurological signs 38 and 42 months after injection and the two animals injected with BSE material developed neurological signs after 46 and 47 months. Post mortem examination of the brains revealed spongiform encephalopathy especially in the basal nuclei and diencephalon of all the animals and, in addition, involvement of the cerebral cortex of the marmosets injected with scrapie material. The experiment extends the host range of experimental BSE to include a primate species.