Four new type C viruses were isolated from putatively virus-negative baboon lung, kidney, and testicular cells by cocultivation with several permissive host cell lines. The baboon type C viruses are infectious for cells from various mammalian species, but do not replicate in any baboon cell lines so far tested. These viruses can be distinguished from other major classes of mammalian type C viruses, including previous isolates from primates, but are most closely related to endogenous feline viruses of the RD-114/CCC group. By immunologic criteria, viral host range, and nucleic acid hybridization studies, the baboon type C viruses are highly related to one another and represent a distinct new class of endogenous primate type C viruses.