Cytotoxic effects, chromosomal aberrations and malignant transformation in a cloned rat liver cell strain(Ac2F) were examined by treatment with an active metabolite of 3'-methyl-4-dimethylaminoazobenzene(3'-Me-DAB), 3'-methyl-N-benzoyloxy-methylaminoazobenzene (3'-Me-N-benzoyloxy-MAB). The results are summarized as follows. 1) 3'-Me-N-benzoyloxy-MAB induced not only marked cytotoxic effects but also chromosomal aberrations on the cells when compared with 3'-Me-DAB. 2) The cells underwent malignant transformation at 51-53 days of culture after they had been treated three times with 6.9 ug/ml of 3'-Me-N-benzoyloxy-MAB. 3) 3'-Me-N-benzoyloxy-MAB-treated cells exhibited altered morphology, increased plating efficiency in liquid medium and increased size of aggregates in rotation culture, but not colony formation in semisolid agar medium or multinucleation by cytochalasin B. 4) These results strongly suggest that 3'-Me-N-benzoyloxy-MAB was an ultimate type carcinogen of 3'-Me-DAB.