The degree and duration of immunity induced by injections of killed poliomyelitis vaccine were studied in 4,000 children. A smaller serologically negative group received special study. Antibody titers were measured before and after the primary immunization, before and after the first booster injection, and before and after the second booster injection. The average immune status was raised by the primary vaccination, but it declined during the ensuing year, was raised to a higher level by the first booster, declined less thereafter, and reached its highest level after the second booster. The response to type 2 antigen was better than the responses to types 1 and 3. There were indications that the commercial vaccine used in the later phases of the study was more potent than the experimental vaccine used in the beginning. The authors recommend that the primary series of three injections of ultraviolet-irradiated formalintreated poliomyelitis vaccine be followed by at least one and possibly two booster injections at yearly intervals.