Recently, it was discovered that sialic acid residues on group B streptococcal (GBS) capsular polysaccharides (CPS) are O-acetylated. Since GBS vaccine development has focused on de-O-acetylated CPS, it became germane to investigate the influence of de-O-acetylated GBS vaccine formulations on functional activity of sera against strains that bear the O-acetyl modification. Post-immunization sera from healthy adult recipients of de-O-acetylated GBS CPS-tetanus toxoid conjugate vaccines were evaluated in opsonophagocytosis assays using 20 GBS clinical isolates representing type Ia, Ib, II, III, or V CPS that varied in amount of O-acetylation from 2 to 40 percent. Ninety percent or greater opsonophagocytosis and killing of all strains were achieved, using CPS type-specific post-immunization sera. These data indicate that de-O-acetylated CPS-conjugate vaccines contain immunogenic epitopes that offer protection against GBS, independent of O-acetyl CPS modifications. Thus, presence of O-acetyl groups on the GBS CPS is not essential for functional antibodies to be elicited by GBS glycoconjugate vaccines.