Ultrastructural Comparison of a Virus from a Rhesus-Monkey Mammary Carcinoma with Four Oncogenic RNA Viruses

The ultrastructure and morphogenesis of Mason-Pfizer monkey virus, isolated from a mammary carcinoma in a Rhesus monkey, was compared with those of murine mammary tumor virus, murine leukemia virus, L1210 leukemia-associated virus, and avian myeloblastosis virus. The simian virus resembled murine mammary tumor virus and the L1210 virus in that it produced intracytoplasmic particles that were enveloped during budding. It resembled L1210 virus and murine leukemia virus in budding with smooth envelopes. It differed from all the others in being more fragile. These similarities, combined with biochemical characteristics reported elsewhere in this issue, suggest that the monkey virus is an oncogenic RNA virus.