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Oxidative hair dyes consist of two components (I and II) that are mixed before use. Aromatic amines in component I and their reaction with hydrogen peroxide after mixing them with component II have been of primary concern. In addition, two in vitro genotoxicity assays are still required for the approval of the final products of oxidative hair dyes in China, and the substance in the oxidative hair dye that causes the high rate of positive results in genotoxicity tests, especially the Ames test, has not been fully elucidated. In this study, we analyzed the formulation of 55 different oxidative hair dyes from 7 color series and performed Ames tests in the strain TA98 with the S9 mix (oxidative hair dyes No. 1–30) and in strain TA97a without the S9 mix (oxidative hair dyes No. 31–55). We found that toluene-2,5-diamine sulfate (2,5-diaminotoluene sulfate, DATS) in component I may be the cause of mutagenicity in TA98, and hydrogen peroxide in component II may be the cause of mutagenicity in TA97a, and their positive concentrations were consistent with those that we calculated from Ames tests. The results suggest that the positive results for the oxidative hair dye in the Ames test were inevitable because of the existence of DATS in component I and of hydrogen peroxide in component II. Therefore, we should carry out safety assessments on each raw material and carry out risk assessments on the final products of oxidative hair dyes instead of genotoxicity tests in China. [ABSTRACT FROM AUTHOR]