Growth of hematopoietic stem and progenitor cells found in the mononuclear cell (MNC) fraction of human cord blood and bone marrow was evaluated under atmospheres containing reduced (5%) and normal (20%) oxygen tension. Cord blood MNC were grown in suspension and on preestablished irradiated bone marrow stromal layers, whereas bone marrow MNC were used to initiate one-step long-term bone marrow cultures (LTBMC). Reduced oxygen tension resulted in a substantial increase in both the number and frequency of colony-forming cells observed in all three types of long-term hematopoietic cultures (LTHC) studied. At various time points under low oxygen, progenitor cell numbers were as much as 12-fold, 3-fold, and 4-fold higher for granulocyte-macrophage colony-forming units (CFU-GM), erythroid burst-forming units (BFU-E), and granulocyte erythrocyte macrophage megakaryocyte colony-forming units (CFU-GEMM), respectively. In addition to these numerical increases, progenitor cells were maintained for 1-2 weeks longer under low oxygen conditions. Reduced oxygen tension also increased total cell numbers by as much as fivefold in cord blood suspension cultures, but this effect on total cell numbers was less pronounced in cultures containing a stromal layer. The rate of irradiated stromal layer degeneration, as judged by cell counts and microscopic examination, was reduced under low oxygen. Finally, the beneficial effect of reduced oxygen was comparable to the effect of an irradiated stromal layer for maintaining cord blood progenitor cells in LTHC. These results indicate that low oxygen, which better approximates the in vivo environment, enhances the growth and maintenance of both stromal and progenitor cells for a longer period of time in vitro.