During the course of an immune response it has been reported that both symmetric and asymmetric IgG antibodies are synthetized in variable proportions by the single cellular clone. Asymmetric molecules present a carbohydrate group in only one of the two Fab fragment, which induces steric hindrance for the binding to the antigen. It was observed that placental factors as well as multiple injections of Ags in different conditions may modulate immune response, with preferencial synthesis of the asymmetric IgG molecules. The present work was focused on the quality of humoral immune response against tetanus (T) and diphtheria (D) during pregnancy, the effect of vaccination during this period and the transfer of maternal antibodies to the fetus. Serum samples from 30 healthy women, pregnant primipare and multiparae, who received multiples doses of vaccines during previous pregnancies and their new-borns were obtained immediately after delivery and again one month later. Serum samples of 25 non-pregnant women were included as control group. Anti-T and Anti-D antibody levels were measured by ELISA. The proportion of symmetric and asymmetric IgG molecules was determined by Con.A-Sepharose chromatography and their respective protective capacity was evaluated by in vitro assays. The percentage of asymmetric anti-T and antiD antibodies in mothers and newborns at delivery was roughly 40%, four-fold higher than that found in the control group (10-12%). In both groups, these values dropped significantly one month later. At the time of delivery, both mothers and their children showed anti-T levels higher than the threshold of protection (0.1 Ul/ml). For diphtheria, 67% of mothers and their children were protected (antiD levels>0.1 Ul/ml) at delivery, while only 46.7% remained protected one month later. The protective activity of asymmetric antibodies proved four-fold lower than that of the symmetric ones. In addition, a competitive and blocking behavior was observed for asymmetric molecules. Repeated immunizations during pregnancy failed to affect the increase in asymmetric antibodies. Even though a considerable asymmetric proportion was found for both specificities, since maternal anti-T titres were very high, this would not affect infant protection. However, in the case of diphtheria, since very low titres were found, it could compromise the protection of the newborn during the first months of life.