The primary IgM and the secondary IgG antibody responses to pneumococcal capsular polysaccharides type 19 (S19) and type 6 (S6) coupled to sheep erythrocytes (S19-SRBC or S6-SRBC) differ in specificity. Although the primary IgM response appears to be totally specific for the pneumococcal cell wall carbohydrate (PnC) which is present in these polysaccharide preparations, the secondary IgG response appears to be completely specific for the immunizing capsular polysaccharide. A library of B cell hybridomas from fusions of splenocytes undergoing a primary response to an S19 preparation consisted entirely of PnC-specific hybrids. Thus, no evidence was obtained for the presence of capsular polysaccharide-specific IgM secreting B cells. IgM and IgG antibody secreting hybridomas were obtained, from fusions of splenocytes undergoing secondary S6- or S19-SRBC responses, to examine the antigen specificity of secondary antibody response of B cells at the clonal level. Many of the secondary IgM hybridomas secreted PnC-specific antibody; however, several S6-specific IgM secreting hybrids were also obtained, demonstrating a previously undetected population of B cells. All IgG secreting hybridomas obtained from S19- or S6-SRBC secondary response fusions secreted capsular polysaccharide-specific antibody, thus confirming the apparent absence of PnC-specific IgG secreting B cells in these responses. This method of immunization and challenge of mice with capsular polysaccharide coupled to erythrocytes, which results in the production of capsular polysaccharide-specific IgG responses, offers a relatively straightforward means to generate monoclonal antibodies specific for pneumococcal capsular polysaccharides.