Two of eight mutants (bmr) isolated in Saccharomyces cerevisiae on the basis of their increased resistance to lethal effects of antitumor bleomycins (BM), and about two-thirds of 180 yeast mutants (bms) isolated on the basis of their increased sensitivities to cell-killing by phleomycins (PM) or BM were sensitive to one or more of the agents UV, X-rays or hydrogen peroxide. Thus these mutants are likely to be altered in processes acting directly or indirectly on DNA damage. The remaining six bmr mutants and approximately 60 bms mutants appear as resistant as the parent strain to cell-killing by UV or X-rays, and are likely therefore, to be altered in cell wall or membrane function. A genetic basis for the phenotypes of some of the bmr and bms mutants has been established.