The effects of nalidixic acid, mitomycin C, and cadmium chloride $(CdCI_2)$ on the activity of 8-hydroxyguanine $(oh^8Gua)$ endonuclease, a DNA repair enzyme for oxidatively modified guanine, $(oh^8Gua$ were studied. Nalidixic acid and mitomycin C, typical inducers of the S0S DNA repair response in E. coli, showed different effects. Nalidixic acid raised the activity of this enzyme, but mitomycin C did not show such an effect. Cadmium chloride also induced the enzyme activity, These results show that the expression of $oh^8$ Gua endonuclease is regulated by multiple factors and can be induced under stressful conditions. In an attempt to demonstrate the importance of this enzyme in defense against DNA damage and mutagenesis, we also characterized mutM mutant for its oh8 Gua endonuclease activity. The mutM mutant showed no detectable $oh^8$ Gua endonuclease activity, unlike its wild type showing high activity. In addition, paraquat, a superoxide producing compound, failed to elevate $oh^8Gua$ endonuclease activity in this mutant. These results suggest that the mutM gene is identical to the $oh^8Gua$ endonuclease gene of E. coli. Taken together with previous reports, these results suggest that $oh^8Gua$ endonuclease plays a crucial role in the protection of aerobically growing organisms from threats of oxidative DNA damage and mutation.