A radiosensitive DNA repair-deficient xrs-5 cell line was used to study asbestos cytotoxicity and DNA double strand breaks (DSBs). Although xrs-5 cells did not show any increase in sensitivity to chrysotile fibers in short-term (4-h) treatment when compared with wild-type CHO cells, longer-term exposure (24 h) gave significantly lower cell survival accompanied by a cell growth delay as well as a higher DNA DSB induction in this mutant cell line. These results suggest an important role played by DNA DSBs at the initial stage of asbestos injury.