To determine whether tetanus toxoid given during pregnancy increases the risk of haemolytic disease of the new-born, 818 group 0 pregnant women were given one to three doses of tetanus toxoid during the third trimester. There was a significant increase in the incidence of neonatal jaundice among the AB0-incompatible infants of women given toxoid, compared to a control group. Comparative studies were also done at two hospitals: tetanus toxoid was not given during pregnancy at one hospital and was used routinely at the other. More of the mothers given toxoid had high titres of igG anti-A or anti-B and more of their AB0-incompatible infants became jaundiced. Thus these studies suggest an increased risk of haemolytic disease of the new-born due to AB0 incompatibility when tetanus toxoid injections are given during pregnancy.