A serologic survey of the poliovirus immunity of children in a suburban school system was performed in 1964, two years after a mass oral vaccination program. Approximately 6% of the school children were studied and 96% had antibody titers of 8 or greater to all three types of poliovirus. Antibody levels declined during the two-year period, but only significantly so for types 2 and 3 in children who had not received a booster. Antibody titers to types 2 and 3 poliovirus in children who had been given a booster were significantly higher than those who had not received one. A single feeding of a trivalent live oral poliovirus vaccine resulted in a rise in antibody titer in slightly less than half of the subjects with low type 3 titers.