Biphasic kinetics (half-times 7' and 250') adequately describe the aerobic repair of DNA breaks after irradiation of mammalian cells. The proportions repaired by the two components are affected by conditions prior to, during and after irradiation. DNA of cells irradiated in hypoxia have approximately twice as much of the damage which is repaired by the slow component as cells irradiated in air. If, instead of oxygen, misonidazole is present during hypoxic radiation, the repair resembles the repair of oxic damage more closely than repair of hypoxic damage. The biphasic nature of the repair curves is interpreted to be due to two classes of initial damage, proportions of which can be altered by sensitizers such as O2 and misonidazole.