Baboons were given 1200 R total body irradiation from two opposing 60Co sources. Three animals were given supportive therapy only and died, as expected, within 8 days of irradiation with profound marrow hypoplasia. Five baboons were cross-circulated with unirradiated partners and died within 14 days with evidence suggestive of graft-versus-host disease. Their marrows were repopulated with hemopoietic precursor cells, and three of the five had rises in peripheral white blood cell counts to more than 1500/cu mm before death. These results are compatible with the presence of hemopoietic stem cells in the peripheral blood of a nonhuman primate, the baboon.