Antibodies to a capsular polysaccharide (PS) provide protection against Streptococcus pneumoniae which express the homologous capsular serotype, and pneumococcal vaccines are designed to induce antibodies in the capsular PS. Levels and opsonophagocytic capacity of antibodies to the capsular PS of S. pneumoniae serotype 19F were determined by sera from adults immunized with 23-valent S. pneumoniae capsular PS vaccines. Geometric means of IgG anti-19F antibody level and specific opsonic titer rise significantly after immunization. The level of anticapsular PS antibodies for S. pneumoniae 19F serotype is fairly well correlated (r²=O.63) with the opsonophagocytic activities of sera. However, 3.7% (1/27) of serum samples display strikingly less opsonophagocytic activity than expected on the basis of their antibody level. Thus, antibody level may be of general use in predicting vaccine-induced protection among adults for 19F serotype. However, the opsonic activity data suggest that antibody levels are not always indicative of functional antibody.