Mammalian cell lines that are sensitive to particular genotoxic agents have proved the most effective starting point for the cloning of human DNA-repair genes. After ethyl methanesulphonate mutagenesis of the parent murine fibroblast L-cell line, a new mammalian X-ray-sensitive cell line (WMXRS-1) was isolated. For selection of the mutant, a novel detection method was used: putative X-ray-sensitive clones were identified by their lack of incorporation of the DNA precursor, bromodeoxyuridine, after irradiation. The WMXRS-1 cell line was collaterally sensitive to ultraviolet radiation and some other agents known to be removed from DNA by the nucleotide excision repair pathway, but not to bleomycin or hydrogen peroxide. In relation to the wild-type strain, WMXRS-1 showed a similar pattern of induction of micronuclei up to an X-ray dose of 4 Gray and a similar DNA double-strand break (dsb) induction profile. The overall level of dsb rejoining was the same in the parent and mutant lines. However, WMXRS-1 demonstrated a reduced initial rate of dsb-rejoining, perhaps accounting for its radiosensitivity. WMXRS-1 also showed a greater G2 cell cycle phase accumulation after treatment with mitomycin-C. The cross-sensitivity profile and strand-break rejoining deficiency phenotype of WMXRS-1 is unique amongst previously characterised mammalian mutant cell lines.