Opsonophagocytic capacity and concentrations of antibodies to the capsular polysaccharide (PS) of Streptococcus pneumoniae serotypes 6A, 6B, 19F, and 19A were determined for sera from adults immunized with 14- and 23- valent S. pneumoniae capsular PS vaccines. The concentration of anti-capsular PS antibodies strongly correlated (r = .72 to r = .91) with the opsonophagocytic activities. However, 10%-20% of serum samples display strikingly less opsonophagocytic activity for the cross-reactive serotypes than expected on the basis of their antibody concentrations to the cross- reactive serotypes. From 1 poorly opsonic serum sample, κ and λ fractions of anti-6B antibodies were purified by affinity chromatography. κ (but not λ) type antibody was found to have less opsonophagocytic capacity than expected. Thus, ELISA measurements of antibody responses to pneumococcal capsular PSs may not be reflective of opsonophagocytic function of the antibody.