In 75 patients with subacute fulminant hepatitis B (SAFH), HDAg positive liver cells were observed in 14 cases (18.67%) by direct enzyme labelled method. It was found further that HDV (delta factor) infection was an important cause in producing massive necrosis of liver cells and superinfection of HDV with hepatitis B was identified as one of the causes of SAFH. This experiment also showed that the number of HDAg positive cells was in direct proportion to the area of liver necrosis. No obvious lymphocytic infiltrations and cytotoxic phenomena around the necrotic foci and HDAg positive cells could be found, while the cytoplasmic type HDAg positive cells showed distinct degeneration or even atrophy. Hence it was suggested that HDV could be a direct pathogenesis. No obvious characteristic changes in morphology of HDV infection could be found.