To assess an immunization schedule combining oral (OPV) and inactivated poliovirus vaccines (IPV), a clinical trial was conducted in The Gambia, Oman, and Thailand. Children were randomized to receive OPV at birth and at 6, 10, and 14 weeks of age; OPV at birth followed by both OPV and IPV at 6, 10, and 14 weeks of age; or placebo at birth followed by IPV at 6, 10, and 14 weeks of age. Serum specimens were available at 24 weeks for 1291 (77%) of 1685 enrolled infants. In the combined-schedule group, the proportion of children seropositive at 24 weeks was 95%-99% for type 1, 99%-100% for type 2, and 97%-100% for type 3. In The Gambia and Oman, the combined schedule performed significantly better than OPV for type 1 (95%-97% vs. 88%-90%) and type 3 (97%-99% vs. 72%-73%). Across the study sites, IPV given at 6, 10, and 14 weeks of age provided inadequate protection against poliovirus. The combined schedule provided the highest levels of serum antibody response, with mucosal immunity equivalent to that produced by OPV alone.