We investigated changes in radiosensitivity of peritoneal exudate macrophage colony-forming cells (PE-CFC) when exudative peritoneal macrophages were cultured in vitro. The change in the shape of the dose-response curve of PE-CFC to ionizing irradiation was partly dependent on the concentration of oxygen in the gas phase of the incubators. When cells were incubated in an environment containing 20% oxygen, the value of both Dq and D0 for PE-CFC increased. The dose-response curve of PE-CFC cultured for 3 days resembled that of alveolar macrophage colony-forming cells (AL-CFC). The changes in radiosensitivity were accompanied by an increase in the level of three antioxidant enzymes: superoxide dismutase, catalase, and glutathione peroxidase. However, when they were cultured in a 6% oxygen environment, only the value of Dq increased. When alveolar macrophages were incubated in vitro, no significant change in the shape of the dose-response curve of AL-CFC was noted whether they were cultured in gas phase containing either 20 or 6% oxygen. It is concluded that the radiosensitivity of PE-CFC changes when they are cultured in vitro. The increase in D0 appears to be related to the intracellular level of antioxidant enzymes.