Transplacental transmission of hepatitis B virus infection was studied in 51 Senegalese neonates born to mothers who were chronic carriers of HBsAg. 13 mothers were positive for both HBsAg and HBeAg. Mother-to-infant transmission of these two markers was different with 3 children being HBsAg positive and HBeAg negative at birth and 6 being HBsAg negative but HBeAg positive. At birth none of the 51 children had anti-HBc IgM detected by a highly specific enzyme immunoassay, indicating that none had had a primary immune response in utero to HBV infection. These HBV serum markers in newborn infants indicate contamination by maternal blood at delivery rather than active infection in utero. Prophylaxis at birth is advisable in children of mothers who are chronic carriers.