Fecal shedding of virulent revertant polioviruses was examined in isolates from infants previously immunized with > or = 1 dose of orally administered live attenuated polio vaccine (OPV) alone, enhanced-potency inactivated polio vaccine (EIPV) alone, or a combination of both. After administration of OPV alone, vaccine poliovirus serotypes were recovered in feces within 1 week and for as long as 31-60 days in 30%-80% of subjects after 1 or 2 doses and in 30%-50% after immunization with > or = 3 doses. No revertant poliovirus shedding was observed after OPV challenge in subjects immunized previously with > or = 3 doses of OPV. However, fecal shedding of revertant poliovirus after OPV challenge was observed in 50%-100% of subjects previously immunized with > or = 3 doses of the EIPV. These findings suggest that prior immunization with EIPV does not prevent fecal shedding of revertant polioviruses after subsequent reexposure to OPV.