This report demonstrates that oxygen concentration within the physiologic range of 2.5 to 20% controls the pattern of proliferation of human diploid fibroblasts by modulating their response to serum and purified growth factors. Reducing oxygen concentration from 20 to 2.5% increased the division rate and final density of fibroblasts cultured in serum-containing medium. DNA synthesis in response to serum, as well as to EGF and PDGF, was enhanced significantly. Exposing quiescent cells to reduced oxygen enhanced serum-induced DNA synthesis in a time-dependent manner. The stimulatory effect persisted when the oxygen concentration was raised to ambient levels before the addition of serum. These results suggest that oxygen concentration within the physiologic range may control proliferation indirectly by altering the activity of a stable intermediate that regulates the cellular response to growth factors.