We have studied the kinetics of chromatid aberrations in cells of the Chinese hamster ovary (CHO-K1) derived, X-ray sensitive cell line xrs 5 irradiated in the G2 phase at 37 degrees C, as well as during a cell cycle extended by transient hypothermia at 33 degrees C. While a given X-ray dose was estimated to produce about 4 times as many chromatid break and twice the frequency of exchanges in xrs 5 cells as in the parent line, there was no difference between the lines in the rates of disappearance of chromatid breaks during G2 at either temperature; and similar patterns of chromatid exchange kinetics were observed in the two lines. Both the frequencies and distributions of chromatid breaks at different times after irradiation are consistent with the view that the disappearance of these during incubation represents a repair process. These results imply that the G2 chromosomal radiosensitivity of the xrs 5 mutant residues at the level of initial chromatid damage.