We studied adaptive response in CHL cells by benzidine dihydrochloride, a derivative of benzidine, which was a major mutagenic agent in dye industry. Chromosome aberration analysis was used for the identification of adaptive response to this mutagen. Adaptive and reactive doses were confirmed by cell proliferation rate curve. Cell proliferation rate curve was obtained from the mitotic indices of cells treated with various concentrations of benzidine dihydrochloride for 24 hours. Marked adaptive responses to benzidine dihydrochloride in the induction of chromosome aberration were observed in CHL cells by pre-treatment with low concentrations of benzidine dihydrochloride (0.0047 mg/$mell$ or 0.0094 mg/$mell$) for 24 hours following post-treatment with high concentrations (0.0187, 0.0375, 0.075, 0.15 mg/$mell$) for 24 hours. These adaptive responses were found mostly in the type of chromatid breaks and chromatid exchanges. There is no difference in these results between two adaptive doses, 0.0047 mg/$mell$ and 0.0094 mg/$mell$. The amount of adaptive response, however, was dependent on post-treatment doses.