After immunization with recombinant hepatitis B vaccine, 19 infants were tested serially for hepatitis B surface antigen (HBsAg); 65% of infants had test results positive for HBsAg at least once, the incidence peaking 2 to 3 days from the time of immunization. The longest documented duration of antigenemia was 8 days; in all patients, HBsAg test results were negative by 18 days.