We studied the effects of pharmacologically attainable concentrations of interferon-alpha (IFN-alpha) and gamma (IFN-gamma) on the growth of cells incubated under hypoxic conditions (2% O2; approximately 14 mm Hg partial pressure) or exposed to oxygen at atmospheric pressure (21% O2; approximately 147 mm Hg). The cells were from four IFN-sensitive lines: A-549 lung carcinoma and G-361 human melanoma cells grow better under hypoxic conditions, but the growth of Hep-2 laryngeal carcinoma and WISH amnion cells is not affected by the environmental oxygen tension. The antiproliferative effects of the IFN were assessed in terms of cell cloning efficiency and also from the number of cells, relative to controls, measured 1, 2, and 3 days after seeding. Under hypoxic conditions, the cloning efficiency of A-549 and G-361 cells was increased, and they became significantly less responsive to the antiproliferative effect of IFN, and especially of IFN-gamma. No such effects were seen with WISH or Hep-2 cells. Hypoxic conditions are found in the necrotic areas present in most solid tumors, and our results suggest that these may decrease the antiproliferative effects of IFN. They may in part explain why IFNs have so little antitumor activity in such tumors, and they also suggest methods that may increase this activity.