Studies were carried out to test the hypothesis that inflammatory liver disease increases the expression of specific cytochrome P-450 isoenzymes involved in aflatoxin B1 (AFB) activation. The immunohistochemical expression and localization of various human cytochrome P-450 isoforms, including CYP2A6, CYP1A2, CYP3A4, and CYP2B1, were examined in normal human liver and liver with hepatitis and cirrhosis. The constitutive expression of CYP3A4 in normal liver showed a characteristic pattern of distribution in centrilobular hepatocytes, whereas CYP1A2, CYP2A6, and CYP2B1 were expressed uniformly throughout the liver acinus. In sections of liver infected with hepatitis B virus (HBV) or hepatitis C virus (HCV), the expression of CYP2A6 was markedly increased in hepatocytes immediately adjacent to areas of fibrosis and inflammation. CYP3A4 and CYP2B1 were induced to a lesser degree, and expression of CYP1A2 was unaffected. In HBV-infected liver, double immunostaining revealed that overexpression of CYP2A6 occurred in hepatocytes expressing the HBV core antigen. In HCV-infected liver, CYP2A6, CYP3A4, and CYP2B1 were overexpressed in hepatocytes with hemosiderin pigmentation. These results suggest that alterations in phenotypic expression of specific P-450 isoenzymes in hepatocytes associated with hepatic inflammation and cirrhosis might increase susceptibility to AFB genotoxicity.