The response to ionizing radiation of synchronized and plateau-phase populations of xrs-5 cells was studied at the DNA and cellular level. Induction and repair of DNA double-strand breaks (dsb) were measured by the non-unwinding filter elution technique. Biphasic survival curves were obtained throughout the cell cycle as well as in the plateau phase, suggesting the presence in the cultures of a radiation-sensitive and a radiation-resistant subpopulation. Delayed plating of plateau-phase xrs-5 cells did not significantly modify cell survival indicating a deficiency in the repair of potentially lethal damage (PLD). Post-irradiation treatment with araA further indicated a deficiency in the repair of a form of PLD sensitive to this drug in the radiation-sensitive subpopulation. The radiosensitivity of synchronized populations decreased as cells progressed through S. Cells in late S were found to be most resistant to radiation, whereas cells irradiated in G2 + M had a sensitivity similar to that of cells irradiated in G1. The decrease in radiosensitivity in S was tentatively attributed to an increase in the proportion of radioresistant cells in the cell population. Preliminary analysis of the results obtained, assuming the presence of two subpopulations showing an exponential response as a function of dose, indicated no change in the D0 values of the radiation-sensitive and the radiation-resistant component throughout the cell cycle. The D0 value of the radiation-resistant component was similar to the D0 of repair-proficient CHO cells. At no stage in the cell cycle were shoulder-type survival curves observed. Transient expression of the xrs-5 repair gene may underlie the observed variation of radiosensitivity throughout the cell cycle. This gene expression could occur either at random, in a subpopulation of cells independent of cell cycle phase, or in a specific manner related to gene replication and the hemimethylated state transiently found immediately thereafter. It is proposed that the resistant tail observed in the survival curves throughout the cell cycle is due to the former phenomenon, and the decreased radiosensitivity in late S to the latter. xrs-5 cells were found to be deficient in dsb repair in the plateau phase of growth, as well as in G1 and mid-S phase. The latter observation indicates that the reduction in radiation sensitivity in S is not associated with a modulation in the ability of cells to repair dsb.