Following immunization with hepatitis B vaccine, 39 infants were followed prospectively for hepatitis B surface antigen (HBsAg). A total of 69.2% of the infants tested positive for antigenemia at least once. Antigenemia was identified most often at 2-3 days (43.5%) and 5-6 days (43.5%) after immunization. The longest documented duration of antigenemia was 21 days. In all cases the antigenemia was transient and cleared by the 28th day post-vaccination.