Treatment of Salmonella typhimurium and Escherichia coli cells with low doses of hydrogen peroxide results in the induction of thirty proteins and resistance to killing by higher doses of hydrogen peroxide. The expression of nine of the hydrogen peroxide-inducible proteins, including catalase, glutathione reductase and a novel alkyl hydroperoxide reductase is controlled by the positive regulator oxyR. OxyR is homologous to the LysR-NodD family of bacterial regulatory proteins and binds to the promoters of oxyR-regulated genes. The oxidized but not reduced form of the OxyR protein activates transcription of oxyR-regulated genes in vitro suggesting that oxidation of the OxyR protein brings about a conformational change by which OxyR both senses and transduces an oxidative stress signal to RNA polymerase.