Erythropoiesis is increased in cultures of human blood progenitors when oxygen tension is reduced from 20% (room air) to 5% (low oxygen, closer to physiological bone marrow levels). The effects of low oxygen onγ-globin synthesis and colony growth in methyl cellulose cultures of blood mononuclear cells from normal individuals and patients with sickle cell diseases were examined. Low oxygen increased colony numbers by 1.5- to 2-fold and erythropoietin sensitivity by almost 2-fold. The interval required for maximal colony growth in cultures from patients with sickle cell disease (sickle colonies) was reduced from 17 days in 20% oxygen to 13 days in 5% oxygen. Relative synthesis ofγ-globin was examined by labeling with -3H-leucine and electrophoresis on Triton acid urea poly-acrylamide gels. The %γwas 1.7-fold higher in normal and 1.4-fold higher in sickle cultures on day 13 in low oxygen. On day 16 the expected temporal decline was not seen in low oxygen, and the %γwas 2-fold higher in normal and 1.8-fold higher in the sickle studies. Hemin increased colony growth andγ-globin synthesis in normal cultures in air, and the effects of hemin and low oxygen were additive. In sickle cultures, hemin and low oxygen had additive effects on colony growth, but only low oxygen increasedγ-globin synthesis. Interleukin-3 increased colony numbers on day 13, primarily by acceleration of peak growth. Interleukin-3 also increasedγ-globin synthesis in low oxygen in normal but not sickle cultures. Thus, low oxygen increasesin vitrosensitivity to erythropoietin, colony numbers, and relativeγ-globin synthesis in normal and sickle cultures.