The owl monkey (Aotus trivirgatus) has been shown to be an excellent model for studies of oncogenic and non-oncogenic viruses. Studies at this institution have been primarily concerned with Herpesvirus saimiri, the Epstein-Barr virus, H tamarinus, and H simplex. These studies have shown that H saimiri is oncogenic when inoculated into primates, that the malignancy induced by H saimiri can be naturally transmitted from the squirrel monkey to the owl monkey, that in vitro pathogenicity of H saimiri can be modified by passing the virus in a nonpermissive cellular host, that the owl monkey is a useful model for studying the oncogenicity of the Epstein-Barr virus, and the fatal disease induced in owl monkeys by H tamarinus and H simplex can be prevented by vaccination with nonpathogenic variants of these viruses.