ABSTRACT: The active immune responsiveness of the offspring of pregnant mice stimulated with heterologous protein antigen was investigated by measuring the plaque-forming cells (PFC). Mice (C57BL/10;B10) immunized once in pregnancy with ovalbumin (OVA) in the form of Al(OH)3 gel (in alum) or in a soluble form (in saline) developed no anti-OVA PFC response. The anti-OVA PFC response suppression induced in the offspring was high in the offspring of alum-treated mothers and low in those of saline-treated mothers. The optimal dose of OVA in alum that induces the highest immunological memory in pregnant mice caused the complete suppression of PFC development in their offspring. The same dose of OVA in saline induced a negative immunological memory in pregnant mice and partial suppression in the offspring. On the other hand, mice primed prior to conception and boosted during pregnancy developed anti-OVA PFC in significant numbers, and only a partial suppression was established in their young. Based on these data, we discussed the possible mechanisms concerned with the specific suppression induced in the young B10 mice stimulated by OVA.