Delta antigen (delta-Ag), hepatitis B surface antigen (HBsAg) and hepatitis B core antigen (HBcAg) were examined by immunofluorescence and immunoperoxidase staining in 106 deparaffinized liver biopsy samples from HBsAg-positive patients with acute and chronic hepatitis. The delta-Ag was present in 15 cases (14%), with nuclear positivity varying greatly in intensity and prevalence. Patients with chronic hepatitis associated with delta infection had a histological picture characterized by foci of intralobular inflammation, many apoptotic bodies and shrunken hepatocytes with no satellite signs of inflammation. The histological pattern of delta antigen-positive acute hepatitis was characterized by the presence of a large number of intralobular apoptotic bodies. The inflammatory reaction mainly involved the portal tracts with piecemeal necrosis, but without collagen production. In the same cases the pattern of expression of HBsAg and HBcAg was unusual: in three cases HBcAg and HBsAg were concomitantly present, whereas in one case none of the hepatitis B virus markers was detectable.