When UV-exposed cells of Escherichia coli strain B/r were incubated in hydrogen peroxide and yeast extract, a marked lethal effect was observed within 4 min. Previous photoreversal treatment (eliminating most pyrimidine dimers) did not reduce the amount of killing produced by the peroxide. Similar results were observed when the cells were incubated with yeast extract previously irradiated with UV. This effect was eliminated by the presence of the peroxide decomposing enzyme, catalase, during the UV-irradiation of the yeast extract. A slight lethal effect of peroxide-containing medium on unirradiated control cultures was seen. This was increased considerably if the cultures were exposed to photoreversing light prior to incubation with peroxide. The data suggest that some component both of the UV-light and (to a lesser extent) the “white” light from the photoreversing source induces some sublethal damage to the bacteria which renders them susceptible to the lethal effect of subsequent incubation with peroxide. The mutagenic effect of the peroxide was determined at concentrations which produce considerable killing (by utilization of induction of reversion to prototrophy in a mfd + and a mfd − double auxotrophic strain of E. coli). Incubation of unirradiated cultures in peroxide-containing medium caused no measurable modification of the mutation frequency. Incubation of the UV-exposed mfd + strain with peroxide resulted in an immediate and marked decline in mutation frequency which was not seen with the mfd − strain. No increase in mutation frequency of the mfd − strain was seen with incubation in peroxide suggesting that the lethal interaction of peroxide with the UV-damaged cell does not promote in surviving bacteria an increase in mutation frequency for this type reversion to prototrophy.