There is a clear need for a standardized assay for the evaluation of licensed and experimental pneumococcal polysaccharide ( PS ) and conjugate vaccines. We have therefore studied various aspects of antibody assay standardization. ELISA was done on 24 pairs of pre and post vaccination sera against nine different pneumococcal types. We also performed opsonophagocytosis assays on 12 serum pairs to measure functional activity against 7 serotypes, and calculated correlation coefficient (r) values versus ELISA. The r values ranged from 0.66 - 0.9. Opsonic and ELISA did not correlates as well for types 19F and 4. Types 18C and 23F correlated the best, with 0.83 and 0.9 correlation coefficients respectively. For type 19F, assays measuring antibodies of higher avidity did not improve the correlation with functional assays. Antibodies in those sera, where ELISA values did not correlate well with opsonic activity, were strongly cross-reactive with heterologous pneumococcal types such as 11A, 12F, 15B, 22F, and 33F. We therefore set up competitive inhibition assay using 22F as the absorbent for the antibody reactive to the "common epitope" shared among most pneumococcal sero-types, except for type 14. The cross-reactive antibody was not absorbed with soluble C PS. Use of 22F PS as an absorbent in the ELISA improves the type specificity of antibody measurements in sera from adults. Lederle 23-valent vaccine contains pneumococcal 17A PS rather than the originally recommended type 17F. The Merck 23 valent vaccine has the 17F PS. We therefore studied the cross- reaction of the antibodies induced by these two PS by performing ELISA and opsonophagocytosis assays on sera from individuals who had received the Lederle or Merck vaccines. Results showed that 17A PS can induce antibodies to both 17A and 17F. Although 17F PS can induce antibodies to both 17A and 17F, 17A induced greater cross-reactivity. Antibodies to 17A or 17F were found to be opsonic for both 17A and 17F strains by opsonophagocytosis assay.