There is a clear need for a standardized antibody assay for the evaluation of licensed and experimental pneumococcal polysaccharide ( PS ) and conjugate vaccines. We have therefore studied various aspects of antibody assay standardization. We recently found that the specificity of the pneumococcal anti-PS ELISA could be improved by combined use of pneumococcal C PS and type 22F PS to remove non-type specific antibodies. The improved specificity was shown by comparison of antibody concentration estimates before and after use of 22Fabsorption to opsonophagocytic titers. Another investigator in the UK has prepared a set of adult post-pneumococcal PS immunization quality control sera for use by different laboratories to standardize and help validate their pneumococcal ELISA methods. Our laboratory was asked to reanalyze 28 of these sera utilizing our 22F absorption method to select 10 to 12 of the sera that were least affected by the 22F absorption, that is, sera with little of the cross-reactive common antiobodies. This study was completed and a recommendation of 12 sera was distributed to other cooperating laboratories. It is important to know whether chemical modifications used to activate and then conjugate pneumococcal polysaccharides to protein carriers results in structural modifications to any of the PS epitopes. The immune response to PS antigens is clonally restricted. We therefore screened sera from normal adults for elevated antibodies to different pneumococcal types. Sixty-one normal adult sera obtained from the NIH blood bank were screened for naturally occurring antibodies against twelve pneumococcal serotypes (1, 3, 4, 5, 6B, 7F, 9V, 12F, 14, 18C, 19F, and 23F). Selected normal sera as well as monoclonal antibodies will be used to study the effects of different conjugation and chemical modifications on various pneumococcal PSs.