Of the 473 employees of a Prague hospital, examined for the presence of hepatitis B surface antigen (HBsAg) and the antibody to it (anti HBs), 23 (4.9%) were shown to have the antigen and 88 (18.6%) to have the antibody. No differences were found in the prevalence of HBsAg and anti HB according to sex and age, but the percentage of persons with antibodies increased with increasing length of occupation. Significantly higher levels of antibodies were found in persons having direct contact with the patients or their excretions (nurses, laboratory assistants). Positive serological findings were more frequent in employees with a history of falling ill with hepatitis after entering employment in the health services than in persons without this history. In the serological evidence of hepatitis B according to work place, those working in biochemical and haematological laboratories and in the blood transfusion department took first place, followed by persons employed in medical and surgical departments.