The results of immunohistochemical and electron microscopic examinations of liver biopsies from 47 patients with chronic delta hepatitis are presented. Delta antigen (HDAg) was detected by immunoperoxidase method in the liver of 35 (74.4%) patients. HDAg was demonstrated in hepatocyte nucleus alone in 22 (62.9%) patients, simultaneously in the nucleus and the cytoplasm in 9 (25.7%), in the cytoplasm alone in 4 (11.4%). In nuclear localization, delta antigen may fill the entire nucleoplasm or only the peripheral zone of the nucleus. When delta antigen fills the entire nucleus, hepatocytes undergo more marked pathological changes than those in which HDAg is localized in the nuclear periphery or those where no delta antigen was found.