Five years after primary infant immunization with trivalent oral poliovirus vaccine, employing either a three-dose primary series as recommended by the U.S. Public Health Service Advisory Committee on Immunization Practices (ACIP) or a four-dose series as recommended by the Committee on Infectious Diseases of the American Academy of Pediatrics. 115 children were serologically tested for persistence of neutralizing antibodies by the microneutralization test. Of the 57 individuals immunized according to the ACIP recommendation, antibody persistence was demonstrated in 92% for type 1 poliovirus, 98% for type 2, and 84% for type 3. Of those 58 individuals originally receiving a four-dose primary infant immunization series, the persistence of antibody was 98% to type 1, 98% to type 2, and 87% to type 3. Twenty-one of 24 negative sera showed neutralizing ability when tested by a more sensitive plaque reduction test. Thus, individuals completing either immunization schedule demonstrated satisfactory persistence of neutralizing antibody to all three poliovirus types over a five-year period.