Herpesvirus saimiri (HVS) is an oncogenic virus for a variety of nonhuman primates. HVS does not produce overt disease upon inoculation in the natural host (squirrel monkey) but consistently induces neoplasms including lymphomas and lymphocytic leukemias in 4 other species of monkeys. Various drugs inhibit replication of HVS in vitro including cytosine arabinoside and adenine arabinoside. In addition, the lymphoma and leukemia induced in owl monkeys responds to vincristine and prednisolone, cyclophosphamide, cytosine arabinoside, and human interferon. Of the various chemical carcinogens studied, the antitumor agent procarbazine induces neoplasms in a variety of species including monkeys. Thus far this compound has induced acute myelogenous leukemia (AML), lymphoma, and hemangiosarcomas in macaques. We have induced primary liver tumors in macaques with several nitrosamines and aflatoxin B1 and these tumors produce alpha-fetoprotein (AFP) which can be assayed for both diagnosis and therapy. Thus far, therapy of hepatocellular carcinoma has been most successful with surgical resection; and the tumor mass and serum AFP have been less responsive to single agent chemotherapy. These nonhuman primate models are useful for an understanding of the cause, diagnosis, prevention, and treatment of the human disease.