Thirteen of 18 owl monkeys inoculated with intact Herpesvirus saimiri (HVS) developed lymphomas with a protracted clinical course. Lymphadenopathy and splenomegaly were apparent 40-177 days postinoculation in affected monkeys which survived 56-203 days. Lymphocytic leukemia was present in 10 of the 13 affected animals, and actively motile lymphocytes were, present in peripheral blood of all affected monkeys. Lymphomas were also induced in 2 of 2 owl monkeys inoculated with repeated injections of heat-inactivated HVS, and in 2 of 3 monkeys inoculated with HVS-induced lymphoma cells.