Hexasaccharide (HS) containing 3 U of cellobiuronic acid was isolated from Streptococcus pneumoniae type 3 capsular polysaccharide S3 and coupled to bovine serum albumin (BSA), keyhole limpet hemocyanin (KLH), or tetanus toxoid (TT). The immunogenicity of these HS-protein conjugates in BALB/c mice was studied by measuring the production of circulating antibodies and the induction of protective immunity to viable S. pneumoniae type 3. Immunization of BALB/c mice with 0.5 jig of S3 resulted in the induction of immunoglobulin M (IgM) antibodies and complete protection against 25 U of a mean lethal dose of S. pneumoniae type 3 for 19 weeks after immunization. BALB/c mice immunized with 100 ,ug of HSq-BSA (containing 12 ,ug of HS) were also protected due to circulating IgM antibodies. Repeated injections with either 100 ,ug of HSq-BSA (three immunizations) or 100 ,ug of HS6-KLH (two immunizations) resulted in high levels of circulating IgG antibodies. These HS-protein conjugates induced complete protection which lasted at least 14 (HS9-BSA), 23 (HS6-KLH), or 8 (HS16-TT) weeks after the last immunization. Protection against viable S. pneumoniae type 3 could be passively transferred to nonimmunized mice by antisera containing IgM or IgG antibodies or both. Sera containing both IgM and IgG antibodies gave better protection than sera containing only IgM antibodies. The specificity of the induced protection was confirmed by challenge with the non-cross-reacting S. pneumoniae type 11. Anti-capsular polysaccharide antibodies confer immunity to infections with Streptococcus pneumonia (10). These anti-polysaccharide antibodies exert their protective effect by opsonization. S. pneumoniae infections can be prevented by vaccination with purified capsular polysaccharides, and in 1977 a polyvalent pneumococcal polysaccharide was licensed for general use in the United States (1). Because vaccination aims at long-lasting protection against infectious

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A semisynthetic vaccine to Streptococcus pneumoniae type 3 has been developed. The hexasaccharide [3)GlcpA,(l4)Glcp(1]3 was isolated from a

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