Liver tissue from a hepatitis B antigen (HB Ag) carrier and a patient with HB Ag-positive chronic aggressive hepatitis and cirrhosis was examined for HB Ag. The antigen was detected in the nuclei of many hepatocytes by direct fluorescence antibody technique. Uniform 20 nm particles were seen in hepatocytic nuclei under the electron microscope. The particles were aggregated and surrounded by ferritin granules using the indirect ferritin-labeled antibody technique with specific antiserum to HB Ag. These findings indicate that the 20 nm particles carry at least one antigenic determinant of HB Ag.