Ninety two owl monkeys Aotus trivirgatus and 15 squirrel monkeys Saimiri sciureus that were inoculated in 1978 with JC virus, a human polyomavirus or control material have been maintained, monitored and removed from the study after 36 months (August, 1981). To date 13 owl monkeys and 3 squirrel monkeys have developed intracerebral neoplasms.