A survey of 4,452 pregnant women taken to find hepatitis B surface antigen revealed 28 asymptomatic chronic carriers. At birth, 16 of 17 infants studied were negative for HBsAg, whereas anti-HBc was present in all patients at a titer similar to that of the mother. Twelve children were followed for 6 to 18 months. In four of them, HBsAg remained negative and anti-HBc titers progressively decreased and were undetectable when tested after the age of 6 months. In eight infants, HBsAg became positive after an average time of 48 days. Elimination of HBsAg occurred in seven infants; five of them had clinical and biological manifestations of mild hepatitis. Sequential determinations of total complement and C components in three patients of the latter group showed dperession of complement at the time of appearance of clinical manifestations. In the patient who became an HBsAg carrier as well as in three infants who remained HBsAg negative, no decrease in complement titers was observed. These results indicate that vertical transmission from carrier mothers can occur in a low prevalence area and that neonatally infected children are capable of active elimination of HBV.