Antibody responses to poliovirus type 3 were studied in fecal samples of 66 children immunized with 3 doses of enhanced-potency inactivated poliovirus vaccine (E-IPV), followed by 1 dose of monovalent oral poliovirus vaccine (OPV, type 3 Sabin). One fecal sample taken before OPV vaccination and 9 collected thereafter were tested for neutralizing antibodies by a microneutralization assay and for class-specific responses by heavy chain-capture radioimmunoassays. Both neutralizing antibody and IgA responses usually occurred during the second week and coincided with ceasing of virus excretion or a decrease in the excreted virus titer. Half of the vaccinees had received a trypsin-modified E-IPV, but their responses did not differ from those of children immunized with the regular E-IPV. These results are in agreement with the view that an intestinal antibody response, mainly consisting of IgA, may be involved in the ceasing of a primary poliovirus excretion.