m-Phenylenediamine (PD) is one of the components of hair dye used in our country. In order to evaluate the modulating effect of other hair dye components on hydrogen peroxide (H₂O₂) oxidation of m-PD, 8 kinds of phenol derivatives and p-PD were treated with H₂O₂ in the presence or absence of m-PD. Before and after treatment with H₂O₂, their mutagenicity was tested by using Salmonella typhimurium TA98 with or without S9mix. The mutagenic potency of m-aminophenol (AP) was most remarkably enhanced with H₂O₂ among 8 phenol derivatives tested. p-AP and phloroglucinol (PH) were slightly mutagenic in the presence of S9mix after treatment with H₂O₂, but the other phenol derivatives were not mutagenic with or without S9mix both before and after treatment with H₂O₂. As major mutagenic products after the H₂O₂ oxidation of the mixture of m- and p-PD, 2, 7-and 2, 8-diaminophenazine (diNH₂-Pz) were identified by TLC and HPLC comparing with authentic samples. The yield of 2, 7-diNH₂-Pz obviously decreased with an increase in p-PD, but the formation of 2, 7-diNH₂-Pz was not completely inhibited by the addition of a 9-fold molar ratio of p-PD to m-PD. The mutagenic potency of the oxidation mixture obtained from m-PD and phenol derivatives decreased and the production of 2, 7-diNH₂-Pz from m-PD was inhibited by the addition of phenol derivatives to the m-PD oxidation system. From the HPLC analysis of the oxidation mixture, the declined mutagenic potency of the oxidized mixture from m-PD and phenol derivatives was found to be mainly due to a decrease in 2, 7-diNH₂-Pz. The modulating effect of m-AP on the oxidation of m-PD was the strongest among 8 phenol derivatives tested.