We have found that Bloom's syndrome (BS) cells exhibit elevated levels of superoxide dismutase activity. Since SOD activity has been shown to reflect the intracellular superoxide (O2-) content, these results indicate that BS cells exhibit oxidative stress which ultimately results in DNA damage. Elevated sister chromatid exchange, the major cytological characteristic of BS, and superoxide dismutase induction were simulated in normal lymphoblastoid cells by treatment with compounds that increase the steady-state concentration of O2(-.). The sister chromatid exchange response of a BS lymphoid cell line was modulated through the control of the endogenous O2-. content. We therefore suggest that a major biochemical defect resulting from this genetic disorder is chronic over-production of the superoxide radical anion. The consequence of high O2-. levels concomitant with induced superoxide dismutase activity is the formation of enormous amounts of H2O2 which can apparently inactivate the enzymes responsible for its elimination. The inefficient removal of peroxide can result in high rates of sister chromatid exchange and chromosomal damage in BS cells and in normal cells treated with oxidation-reduction cycling compounds through the formation of highly reactive intermediary forms of active oxygen.