In Escherichia coli hosts, hydrogen peroxide is one of the factors that may cause induction of lambda prophage. Here, we demonstrate that H2O2-mediated lambda prophage induction is significantly enhanced in the oxyR mutant host. The mRNA levels for cI gene expression were increased in a lambda lysogen in the presence of H2O2. On the other hand, stimulation of the p(M) promoter by cI857 overproduced from a multicopy plasmid was decreased in the DeltaoxyR mutant in the presence of H2O2 but not under normal growth conditions. The purified OxyR protein did bind specifically to the p(M) promoter region. This binding impaired efficiency of interaction of the cI protein with the OR3 site, while stimulating such a binding to OR2 and OR1 sites, in the regulatory region of the p(M) promoter. We propose that changes in cI gene expression, perhaps in combination with moderately induced SOS response, may be responsible for enhanced lambda prophage induction by hydrogen peroxide in the oxyR mutant. Therefore, OxyR seems to be a factor stimulating lambda prophage maintenance under conditions of oxidative stress. This proposal is discussed in the light of efficiency of induction of lambdoid prophages bearing genes coding for Shiga toxins.