The livers of 49 persons who died from different forms of hepatitis were examined by immunofluorescence (IFL) and electron microscopy. Hepatitis-associated antigen (HAA) was found by IFL in 40% of cases of subacute hepatitis, 54% of cases of chronic aggressive hepatitis, and 31.6% of cases of postnecrotic cirrhosis of the liver. The antigen was not found in any of the nine cases of acute fatal and fulminant hepatitis. Typical HAA particles were identified in the nuclei of hepatocytes in all cases of chronic aggressive hepatitis and postnecrotic cirrhosis studied by electron microscopy. Combined immunofluorescent and histologic studies revealed an inverse correlation between the amount of HAA and the extent and severity of parenchymal changes in the liver and the presence of HAA in body fluids. The results provide additional evidence that subacute hepatitis and the different types of chronic hepatitis, including hepatic cirrhosis, are etiologically related to HAA.