Attenuated poliovirus vaccine (Sabin) was given to 90 young infants by 5 schedules of administration in order to determine the degrees of immunologic response and the influence of placentally transferred passive antibodies. Group 1 received 3 vaccine types at 6-week intervals as monovalent vaccine; Group 2 received 3 types administered together on 3 separate occasions; Groups 3, 4, and 5 received the 3 types as a single dose at 6 weeks, 12 weeks, and 18 weeks of age, respectively. A sixth group, as a contratest, did not receive vaccine. Groups 1 and 2 developed the highest antibody levels. High levels of passive antibody of maternal origin did not interfere with production of active antibody.