The radiosensitive Chinese hamster cell line XR-V15B was used to study the effect of decreased rejoining of DNA double-strand breaks (DSBs) on gene mutations and chromosome aberrations. XR-V15B cells are hypersensitive to the cytotoxic effects of neocarzinostatin (NCS) and methyl methanesulfonate (MMS). Both mutagens induced more chromosome aberrations in XR-V15B cells than in the parental cell strain. The clastogenic action of NCS was characterized by the induction of predominantly chromosome-type aberrations in cells of both strains, whereas MMS induced mainly chromatid aberrations. The frequency of induced gene mutations at the hprt locus was not increased compared to the parental V79 cells when considering the same survival level. Molecular analysis by multiplex polymerase chain reaction (PCR) of mutants induced by NCS revealed a high frequency of deletions in cells of both cell lines. Methyl methane-sulfonate induced mainly mutations without visible changes in the PCR pattern, which probably represent point mutations. Our findings suggest a link between a defect in DNA DSB repair and increased cytotoxic and clastogenic effects. However, a decreased ability to rejoin DNA DSBs does not seem to influence the incidence and types of gene mutations at the hprt locus induced by NCS and MMS.