The frequency of exchanges in JU56 cells irradiated in the G2 phase in the presence and absence of the polymerase inhibitor aphidicolin (APC), and in the presence of a range of concentrations of cysteine was measured. It was found that in the absence of cysteine, incubation for 2 h with APC had no effect on the yield. Addition of cysteine at concentrations of 50-250 mg/l reduced the frequency of exchanges, and at these concentrations the frequency was increased by incubation with APC. At higher concentrations, the yield was reduced and incubation with APC did not elevate it. In following experiments, it was found that incubation with cysteine for a period of longer than 10 minutes was necessary before APC affected the yield of exchanges.