Prior to hepatitis B vaccination, 36,000 persons of the medical staff were tested for HBs antigen, HBc antibodies, and HBs antibodies. 210 sera were found positive for HBs antigen and HBc antibodies. Of these sera, 171 were available for testing for hepatitis B virus DNA as a marker of infectivity by spot hybridization. DNA was detected in only 15. One hundred and thirty-nine had HBe antibodies but no detectable HBe antigen, and only two of these were hepatitis B virus DNA positive. 12 had neither HBe antigen nor HBe antibodies and none had hepatitis B virus DNA. Hepatitis B virus DNA was, however, detected in 13 of 20 HBe antigen-positive but HBe antibody-negative sera. Our study confirms epidemiological observations that medical staff hardly plays any role as a source of HBV infection for patients.