The mutagenicity of 3 dihydroxybenzene (DHB) and 9 dihydroxynaphthalene (DHN) isomers was examined by using 5 different Ames Salmonella mutagenicity tester strains in the presence and absence of phenobarbital and 5,6-benzoflavone-treated rat liver S9-mix. Of the 3 DHB isomers, 1,4-DHB (hydroquinone) was mutagenic, and of the 9 DHN isomers, 1,3-DHN (naphthoresorcinol), 1,4-DHN (hydronaphthoquinone), 1,6-DHN and 1,7-DHN were mutagenic. Mutagenicity of all the compounds tested was observed in the absence of S9-mix, while 1,4-DHN and 1,6-DHN were also mutagenic in the presence of S9-mix. The mutagenicity of 1,4-DHB and 1,4-DHN for TA104, which is a strain sensitive to oxidative mutagens, was almost completely or partially inhibited by superoxide dismutase (SOD) and/or catalase, indicating the involvement of activated oxygen species in mutagenesis. Furthermore, from the finding that the 4 DHNs were mutagenic for TA2637, the strain sensitive to frameshift mutagens, it is possible that the mutagenicity of DHNs for S. typhimurium was also attributable to DNA adducts that form with quinones and/or semiquinones through oxidation of DHNs. The mutagenicity of 1,3-DHN, which showed the largest number of revertants in strains TA100, TA98, TA2637 and TA104, was greatly decreased, when their pKM101 plasmid-deficient strains, TA1535, TA1538, TA1537 and TA2659 were used. This observation suggests that an SOS repair system was involved in the mutagenesis of 1,3-DHN for S. typhimurium.