Nineteen bleomycin-sensitive Chinese hamster ovary cell mutants have been isolated using a replica plating and photography approach. As judged by the dose which reduces cell survival to 37% of the untreated control, these mutants are from 2.5- to 32-fold more sensitive to a 16-h bleomycin treatment than the parental cell, while for chronic bleomycin exposure, the increase in sensitivity was 5 to 58 times that of the parental cell. Four bleomycin-sensitive mutants had increased sensitivities to killing by gamma-rays (2- to 3-fold), mitomycin C (2-fold), and ethyl methane sulfonate (4- to 5-fold), while six other mutants were resistant to these agents. Nine other bleomycin-sensitive mutants displayed a variable pattern of cross-sensitivities to these agents. Using the technique of alkaline elution, the relative frequency of single-strand DNA breaks introduced by varying concentrations of bleomycin was examined in one mutant and its parent cell. The elution profiles of both cells were similar, suggesting that the bleomycin sensitivity of this mutant is not due to a greater frequency of single-strand breaks introduced by bleomycin.