Female ICR mice were vaccinated against Toxoplasma gondii(RH strain), infected 2 weeks later and after recovery mated to normal ICR males. Control matings were with normal ICR females. The progeny of the above matings were weaned at 1 week, vaccinated at 1, 2, 3, 4 or 5 weeks of age and infected 2 weeks later with lethal Toxoplasma tachyzoites. As assessed by survival, the effectiveness of vaccination among offspring of vaccinated-recovered mothers was greatly impaired than that of control mice, with respective of age : where mice did survive, recovery was greatly delayed relative to the controls. The protective effect of vaccination among infants born to control mothers was also blocked by maternal specific antibodies, by administration of high-titered specific antibodies or by transfer of nylon wool adherent immune-spleen cells, but was augmented by transfer of nylon wool passed immune-spleen cells. These results indicate that this impairment of vaccination may be due to the transmission of maternal specific antibodies to the offspring which acts to suppress both priming by the vaccine and the generation of parasite-specific helper T cells.