Neonatal tetanus kills 2,00,000 infants annually in India. To simplify the logistics of tetanus immunization in pregnancy a potent single dose vaccine containing 250 Lf tetanus toxoid (TT) was developed. When administered to 200 unimmunized primigravidae in the last trimester of pregnancy, all generated protective titres of tetanus antitoxin (> or = 0.01 IU/ml) at delivery as measured by the mouse neutralization method. All 125 hospital born infants of these mothers had antitoxin 0.01 IU/ml in cord blood. Protective levels of antitoxin persisted in 38 babies who followed up at 3 months and in 25 more seen between 4 and > or = 6 months;--none had received infant immunization. No adverse effects of 250 Lf (TT) were encountered in the mothers and there was no increase in stillbirths. The vaccine appears suitable for mass immunization.