THIS PAPER DESCRIBES: (1) a continuation of the study reported in 1965 of tetanus antitoxin titres among women after primary immunization with plain, AlPO(4)-adsorbed or oil adjuvant toxoids; (2) the effect of age and of abscess formation due to oil adjuvants on antitoxin response; and (3) a comparative study of titres in some women of the study groups who received either a plain or an AlPO(4) toxoid booster injection in pregnancy.The results support our previous recommendation to use aluminium adjuvant toxoid in the prevention of neonatal tetanus. Mean maternal protective antitoxin levels were maintained for 40 months but not for 54 months after 2 primary injections and booster response was higher in those immunized with this toxoid than in those immunized with plain toxoid. Also, the use of AlPO(4) toxoid as booster stimulated a higher antitoxin response than did the use of plain toxoid. Mean protective levels induced by oil adjuvant toxoids (1 injection) persisted for the same time as for the adsorbed toxoid (2 injections). Age did not significantly affect the level of antitoxin response, whereas abscess formation was associated with a higher level.Pregnancy appeared to favour the incidence of antitoxin responders to adsorbed toxoids. Failure to respond to primary immunization with 2 injections of adsorbed toxoid occurred in 7% of women while 100% responded to the oil adjuvant toxoids. Induction of abscess formation precludes use of the latter toxoids, however. Additional studies are needed on the influence of larger antigen doses on the primary response in different ethnic and ecological situations and on the influence of smaller antigen doses on the booster response where repeated booster injections are indicated.