Hepatitis B associated delta infection was found in Australian born narcotics users, patients with cirrhosis and hepatoma from Italy and Greece and in south east Asian immigrants. About 9% of patients with chronic hepatitis B were found to have intranuclear delta antigen in liver biopsies with immunofluorescence or anti-delta antibody in serum by radioimmunoassay. Delta may have been imported from southern Europe and Asia and a reservoir of delta infection may exist in narcotics users. Anti-delta was not found in hemophiliacs despite exposure to hepatitis B. Delta occurs in Australia and causes acute and particularly chronic hepatitis in addition to that caused by hepatitis B.