tert-Butylhydroperoxide (tB-OOH) induces killing and DNA single strand breaks (SSBs) in cultured U937 cells. Pyruvate while increasing the rate of oxygen consumption also increased the magnitude of the DNA scission produced by tB-OOH. Rotenone, an inhibitor of complex I, abolished both effects but did not, however, affect the DNA SSB-frequency observed after treatment with tB-OOH alone. These results collectively suggest that pyruvate potentiates the DNA-damaging activity of tB-OOH via stimulation of oxygen consumption. Importantly, under the same experimental conditions, pyruvate was found to abolish both the decline in nonprotein sulfhydryls (NPSH) and the cytotoxicity induced by tB-OOH. Thus, cells with energized mitochondria are more sensitive to the DNA-damaging effects of tB-OOH and display resistance against its cytotoxic effects. As a consequence, DNA SSBs promoted by tB-OOH do not appear to be toxic for the cell.