The influence of low oxygen tension on the clonal growth of hemopoietic stem cells in vitro was examined. The numbers of colonies of neutrophil, macrophage, and eosinophil progenitors (CFU-C), derived from human bone marrow, increased at a rate 1.7 times higher in low oxygen tension (7% O2) than in a gas phase that contained air (19% O2). The erythroid (BFU-E) and multipotential (CFU-mix) progenitors increased about 2.4 times in 7% O2, and the increase in the composed cell type of mixed colonies showed no rate difference in either gas phase. Under atmospheric conditions, a mouse mast cell progenitor (CFU-mast) formed colonies, with the addition of 2-mercaptoethanol (2-ME). Under low oxygen tension, the CFU-mast formed colonies without 2-ME, but a further enhancement was observed with the addition of 2-ME. Blood gas analysis of human bone marrow showed a pO2 of 51.8 ± 14.5 mmHg, which was closed to O2 tension in a gas phase culture media containing 7% O2. This data shows that the physiological O2 tension enhances hemopoietic stem cell proliferation in vitro, and that a part of the enhancing effect by 2-ME is due to a prevention of O2 toxicity at 19% O2.