Sera were analyzed by radioimmunoassay for type-specific pneumococcal antibody in 249 children aged six to 54 months, who were participating in a controlled trial of a 14-valent pneumococcal vaccine. Levels of serum antibody to all serotypes increased after immunization in all age groups tested. For all serotypes, the antibody response increased progressively with age whether response was viewed as antibody doubling, relative increase in geometric mean, or final antibody level. Responses were poor up to the age of five years for the important pediatric serotypes 6A, 14, 19F, and 23F. Seventeen children under the age of two years at the time of primary immunization received booster doses of vaccine six months later. There was no significant increase in antibody to any serotype, and the geometric mean antibody levels fell for most types. Immune response to the pediatric serotypes was poor until the age of 4.5 years.