AbstractAlthough the vast majority of hepatitis B surface antigen (HBsAg) carriers of the world inhabit South‐east Asia, very little is known about delta infection in this area. Therefore, a serological and immunohistological study was made in the Tokyo‐Chiba area. One of 58 (1.7%) HBsAg carriers had anti‐delta antibody in a high titre in serum. Delta antigen was immuno‐histologically localized in the liver in two of 146 (1.4%) HBsAg carriers studied. The antigen was strongly stained in the nuclei, and positive cells were diffusely scattered throughout the liver in both cases. Neither subject was an illicit drug user: one had travelled to Italy 10 years earlier and the other had a blood transfusion during a 5‐year residence in Bangkok in the past.Thus, there is delta infection among non‐intravenous drug users in Japan. Delta infection has been linked to severe liver damage, occasionally fatal. Once introduced, it could become epidemic in a country where hepatitis B virus infection is endemic, and might spread among non