The aluminium-adjuvanted 11-valent pneumococcal conjugate vaccine containing polysaccharides 1, 4, 5, 7F, 9V, 19F, and 23F (coupled to tetanus protein) and polysaccharides 3, 6B, 14, and 18C (coupled to diphtheria toxoid) elicits high antibody concentrations in Filipino infants when given at ages 6, 10, and 14 weeks and 9 months simultaneously with the national vaccination program. We evaluated functional activity of these antibodies by using a viable cell opsonophagocytic assay (OPA). The OPA titers correlated (r=0.53-0.74) with the respective antibody concentrations. The geometric mean OPA titers after 3 vaccine doses were 276.9 (serotype 4), 12.3 (serotype 6B), 46.0 (serotype 14), 119.3 (serotype 19F), and 206.3 (serotype 23F). The functionality of antibodies increased after the fourth dose of vaccine (i.e., the concentration required for in vitro killing of pneumococci decreased). Both the quantity and quality of antibodies are important in the evaluation of immunogenicity of pneumococcal vaccines.