Types Ia and Ib group B streptococcal (GBS) capsular polysaccharides (PSs) are structural isomers but are antigenically distinct. Immunization of healthy adults with GBS type Ia PS-tetanus toxoid (Ia-TT) or Ib-TT glycoconjugate vaccines induced > or = 4-fold increases in specific immunoglobulin G to the heterologous PS in more than two-thirds of subjects. Ib-TT vaccine-induced IgG bound with substantially higher affinity to homologous (Ib) than to heterologous (Ia) PS and promoted opsonophagocytic killing of GBS type Ib but not type Ia organisms. The failure of the Ib-TT- and Ia-TT-induced human antibodies to kill bacteria of the cross-reactive serotype contrasts with the results of previous studies in animals. Inhibition enzyme-linked immunosorbent assays demonstrated that Ib-TT-induced IgG to the homologous PS bound mainly to native Ib PS, whereas the cross-reactive antibodies recognized both native and derivative PSs. These results indicate that GBS Ia and Ib PSs should be included in a multivalent conjugate vaccine to prevent GBS disease.