The possible involvement of bile duct epithelium (BDE) in chronic hepatitis B was examined by immunohistochemical investigation of HBcAg and HBsAg expression in biliary cells in 47 liver biopsies with both viral antigens detectable in hepatocytes. HBsAg- and HBsAg-positive cells were identified in nine and five cases, respectively, in atypical and occasionally in typical ductules in cases of acute excacerbation, chronic active hepatitis and active cirrhosis. Atypical ductules were usually located in areas of periportal fibrosis and in cirrhotic septa. Liver cell plates expressing viral markers and undergoing ductular transformation (positive reaction of hepatocytes to BDE-specific, wide-spectrum keratin) were also observed in acinar zone 1, at the periphery and within parenchymal nodules in a number of cases. The presence of both viral antigens in atypical ductules in cases of advanced chronic liver disease most probably expresses the persistence of the virus in cells deriving from biliary metaplasia of infected hepatocytes. However, the detection of the virus in a few typical ductules is indicative of a direct viral infection. According to these findings, ductular cells seem to serve as a suitable host for HBV, their genotype permitting viral replication and antigen production.