Hepatitis B virus (HBV) markers were studied by radioimmunoassays in serum samples of 1,200 (647 male, 553 female) apparently healthy children under 15 years of age in Taipei between June and October 1984. The prevalence rate of hepatitis B surface antigen (HBsAg) was 5.1% in infancy, increased to 10.7% between 1 and 2 years of age, and then remained constant at about 10% thereafter. The prevalence rate of surface antibody (anti-HBs), core antibody (anti-HBc), and seropositivity (at least one marker of hepatitis B detectable) were 39.0, 30.5, and 52.5%, respectively, in infancy, then decreased to 10.7, 14.3, and 17.9%, respectively, between 1 and 2 years of age. Thereafter, the antibody prevalence increased in parallel with age. By the age of 13-14 years, nearly half of the children were infected by HBV. The results suggested that in our children, most HBsAg carriers resulted from infections before 3 years of age, and HBV infections after 3 years of age infrequently resulted in a carrier state. One hundred (83.3%) of the 120 HBsAg-positive children had hepatitis B e antigen (HBeAg), indicating high prevalence in young asymptomatic HBsAg carriers. The prevalence rate of HBeAg tended to decrease with age and a reversed trend was observed with anti-HBe. Our study, just before our government extends mass hepatitis B vaccination program from newborns to children, provides background seroepidemiologic data of HBV infections in the healthy children in Taiwan.