The SOS response of Escherichia coli strains carrying the lacZ gene fused to the polB (dinA), dinB or dinD gene were investigated after treatment with several chemical agents and y-radiation. The induction levels of polB∷lacZ reached levels between 4.0- and 9.0-fold 120 min after treatment with nalidixic acid, H₂O₂ or ethanol. Pentachlorophenol did not significantly induce any din genes. γf-Irradiation is not an inducer of polB and ethanol failed to induce polB∷lacZ and polB∷lacZ. Following irradiation with a dose of 10 Gy the responses of dinB and dinD were induced about 2.5 ∼ 3.0-fold above non-irradiated dinB and dinD. We found that the responses of polB∷lacZ fusion genes to these genotoxins are induced in a dose-dependent manner. The polB gene showed antagonistic responses to the simultaneous treatment of nalidixic acid and H₂O₂ or nalidixic acid and ethanol. In addition, dinB and dinD in the presence of both nalidixic acid and H₂O₂ at the same time showed no synergistic responses.