• Sixty-one children who had previously received three doses of enhanced potency inactivated poliovirus vaccine (epIPV) at 2, 4, and 18 months of age and 56 children who had previously received oral poliovirus vaccine (OPV) according to the same schedule were challenged with a single dose of monovalent, type 1 oral poliovirus vaccine (OPV1)between 19 and 52 months of age. Before the OPV1 challenge, the previously epIPV-immunized recipients had a geometric mean poliovirus type 1 microneutralization antibody titer (geometric mean titer [GMT]) of 11.1 IU, which was significantly higher than the prechallenge GMT of 2.2 IU among the children who had previously received OPV. Three weeks after the OPV1 challenge, the GMTs for the epIPV-immunized recipients and the OPV-immunized recipients were 35.3 IU and 5.1 IU, respectively. For the epIPV-immunized recipients, both the prechallenge GMT and the postchallenge GMT were dependent on the D antigen content of the vaccine that they had previously received. A fourfold or greater rise in poliovirus type 1 antibody occurred after the OPV1 challenge in 50.9% of the epIPV-immunized children and in 28.6% of the OPV-immunized children; this difference was statistically significant. For both groups, antibody boosts were inversely correlated with the prechallenge serum antibody titer. However, the epIPV-immunized children consistently were more likely to boost than the OPV-immunized children at equivalent levels of prechallenge antibody. This experience indicated that OPV1 administration effectively raises the level of serum antibody in children previously immunized with three doses of epIPV, especially in children with lower levels of preexisting antibody. This booster response was superior to the booster response of children who received three doses of OPV.