Sister chromatid exchange (SCE) frequency was studied in human/hamster hybrid lines in which the human cells had been exposed to gamma irradiation at doses of 0, 10, 20, and 40 grays. A higher SCE frequency was found in the hamster chromosomes in the hybrid lines than was found in the hamster parental cell line CHW1103. There was also a dose-dependent increase of SCE frequency in the unirradiated hamster chromosomes in the hybrid lines in which the human parental cells had been irradiated.