Employing six forms of cytochrome P-450s fractionated from the hepatic microsomes of PCB-treated rats, the activation of aflatoxin B 1 (AFB 1 ) was examined in the reconstituted cytochrome P-450 system. AFB 1 was specifically activated into DNA-binding form by cytochrome P-450 I-a, which is one of P-450 type cytochromes and possesses an absorption peak at 450.0 nm in its carbon monoxide difference spectrum. This activation was enhanced by cytochrome b 5 and the maximal enhancement (1.6-fold of the control) was observed with the molar ratio of 0.25 cytochrome b 5 :1.0 cytochrome P-450.