Virus characterization studies were performed to meliorate the taxonomic status of three currently unclassified, serologically related viruses: Tanapox virus (causes vesicular skin lesions in humans), Yaba-like disease (YLD) virus (causes vesicular skin lesions in monkeys), and Yaba monkey tumor virus (YMTV, causes epidermal histiocytoma). These studies included (1) microscopic observations of Tanapox virus cytopathic effect and morphogenesis during its 6-day cytolytic-type growth at 35 degrees in CV-1 monkey kidney cells; (2) resolution of Tanapox virion proteins by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of nonenveloped and double-enveloped virus particles purified by velocity sedimentation in sucrose and CsCl density gradients; and (3) restriction endonuclease DNA comparison of the three viruses. DNA analysis showed that six recent Tanapox virus isolates from patients in Zaire, Africa, were identical to Tanapox virus, Kenya strain, from 1957 from a patient in the Tana River Valley. In addition, BamHI, MluI, and PstI cleavage sites mapped on the DNA of Kenya Tanapox virus, and PstI sites mapped on DNA of YLD virus differentiated YLD and Tanapox viruses as separate strains. On the other hand, YMTV shared few restriction endonuclease sites with Tanapox and YLD viruses, although all three cross-hybridized extensively. These studies along with published viral characteristics, support the formation of a new poxvirus genus: the suggested name is Yatapoxvirus, and the genus currently comprises two species, Tanapox virus and YMTV.