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An oncornavirus isolated from a squirrel monkey (Saimiri sciureus) lung culture has a density of 1.16 to 1.17 grams per milliliter, contains 70S RNA, and has an RNA-directed DNA polymerase that prefers Mg2+ over Mn2+ in an assay in which polyribocytidylate - oligodeoxyguanylate (12-18) is used as a synthetic template. Morphologically, the virus resembles Mason-Pfizer monkey virus but is antigenically distinct from this virus. The virus grows in cells of human, chimpanzee, rhesus monkey, canine, and mink origin, but not cells of squirrel monkey origin. On the basis of its properties, the newly isolated virus can be classified as a retravirus.

Ultrastructural investigations have revealed the presence of apparent endogenously derived C-type particles in the placental villi of each of 13 baboons studied. Both budding and mature forms were observed in the syncytiotrophoblast of these animals at various stages of pregnancy.

1 WHO Collaborating Laboratory on Comparative Medicine: Simian Viruses. were listed by The Science Information Exchange of the Smithsonian Institution for fiscal year 1967 at a cost of approximately $55,000,000. Augmented usage of various monkeys and apes may be anticipated for the future, as evidenced by the yearly increase in numbers employed in an attempt to comply with requests by laboratories and by development of additional major primate research centers throughout the world. Breeding programs are anticipated to limit the numbers of animals eventually imported, but such efforts will 310