To investigate the prevalence and distribution of antibodies to hepatitis A antigen (anti HAV Ab) we tested 848 French people living mainly but not exclusively in Franche-Comté (East of France). In Franche-Comté, 76 per cent of all the subjects studied were found to be anti-Hav ab positive, as determined by specific radio-immuno-assay. The rate of antibody detection was correlated with age; the prevalence was low (38 per cent) in childhood and adolescence, gradually increasing throught adulthood, and reaching its peak level (greater than 95 per cent) in people aged 40 and above. Anti-HAV Ab was detected more frequently in farmers (89 per cent), factory workers (87 per cent) and office employees (80 per cent) than in hospital personnel (53 per cent). Socioeconomic differences were found to be important in young people but tended to disappear at about 40 years of age. No difference was found in the prevalence of anti-HAV Ab between national servicemen from Franche-Comté and those from other regions. In 3 per cent of cases anti-HBs Ab was associated with the presence of anti HAV Ab. The subject possessing anti HBS Ab had the same prevalence of anti-HAV Ab (75 per cent) as those without (76 per cent).