Materno-fetal transmission of hepatitis B was studied in 97 healthy carriers of hepatitis B surface antigen (HBsAg). Antepartum transmission occurred in at least 10 per cent. Intrapartum transmission may have occurred in about 40 per cent as a result of swallowing of the infective fluid by the baby during delivery, and materno-fetal transfusion during labour. Person to person transmission after delivery played a minor role. The presence of hepatitis B associated e antigen (HBeAg) in 48 per cent of maternal serum correlated strongly with the subsequent presence of antigen in the infants. There was a linear association between the incidence of antigens in cord blood and the duration of the first stage of labour, with a significant association when labour exceeded nine hours. Caesarean section is recommended if mothers have HBeAg; likewise amniocentesis and breast feeding should be discouraged if mothers have HBeAg.