Opsonin-dependent phagocytosis is believed to be the major defense mechanism against Streptococcus pneumoniae. In addition to type-specific antibodies to pneumococcal capsular polysaccharides (PPS), most persons also produce antibodies to cell wall polysaccharide (CWPS), which may be detected in antibody assays due to contamination of PPS antigens. Antibodies to CWPS and PPS from common serotypes were analyzed in relation to opsonic activity before and after vaccination of healthy adults. A highly significant relationship was generally found between opsonization and pneumococcal antibody levels, and this correlation increased markedly by neutralization of CWPS antibodies. This applied in particular for IgG2, which showed no correlation without neutralization. Adsorption of PPS antibodies almost abolished the opsonic activity, whereas removal of CWPS antibodies had no effect. These findings indicate that opsonization of pneumococci is almost exclusively mediated by PPS antibodies and emphasize the importance of neutralizing CWPS antibodies when immunity to this organism is evaluated.