Leukemia virus transmission was studied during an outbreak of leukemia among the baboons of the Sukhumi monkey colony. First, the possibility of "vertical" transmission of the virus was examined, as the Papio stock in Sukhumi was genetically homogeneous. "Vertical" transmission of the virus from a common ancestor seems unlikely, since cases of leukemia were registered in animals descending from different families. However, there were cases of innate leukemia, when babies born from leukemic parents displayed pronounced signs of leukemia immediately after birth. "Horizontal" transmission of the virus also seems indisputable, because special experiments have shown that imported baboons, even of other species, caged together with leukemic animals, may develop leukemia due to close contact. It was established earlier that the leukemia virus was excreted with urine. Kidneys, lungs, and salivary glands are the possible routes of infection. Thus, the present report aknowledges the possibility of both "horizontal" and "vertical" transmission of leukemia oncornaviruses in primates.