The oxyR-encoded regulatory protein, OxyR, acts to induce the synthesis of a family of hydrogen peroxide-inducible proteins in Salmonella typhimurium and Escherichia coli. To further define the mechanism by which oxyR regulates the production of these proteins, we identified, mapped, and characterized oxyR-regulated promoters upstream from the S. typhimurium ahp genes (encoding an alkyl hydroperoxide reductase) and the E. coli katG gene (encoding catalase). A set of ahpC promoter deletions was constructed in vitro and analysis of these deletions revealed the location of sequences that are involved in oxyR-mediated induction of the ahpC gene product. DNase I protection studies of the ahpC promoter region revealed an oxyR-dependent footprint that overlapped the sequences found to be important for oxyR control. E. coli strains containing transcriptional fusions between the katG promoter and the lacZ gene showed strongly increased synthesis of beta-galactosidase in response to hydrogen peroxide treatment. This stimulation was found to be oxyR-dependent. DNase I protection studies of the katG promoter region revealed an oxyR-dependent footprint in the same location relative to the basal promoter elements as was observed with the ahpC promoter. Although both the ahpC and katG promoters were shown to bind the same factor, no strong sequence similarities were found between the two, or between the two and a third oxyR-dependent binding site upstream from the E. coli oxyR gene itself.