Abstract Sera from rabbits that were hyperimmune to type 3 or type 8 pneumococcal capsular polysaccharide were fractionated by starch block electrophoresis, and fractions containing large quantities of anticapsular IgG of restricted molecular heterogeneity were evaluated for their ability to agglutinate and opsonize encapsulated pneumococci. Specific anticapsular IgG was readily absorbed by the organisms. Each CFU of type 3 organisms was capable of absorbing 5 × 10 7 molecules of IgG, and the type 8 organisms absorbed 5 × 10 6 molecules/CFU. Only small amounts of nonimmune IgG were absorbed. Agglutination occurred only after 10 6 molecules/CFU IgG had been absorbed by type 3 organisms, and 10 4 to 10 5 molecules/CFU had been absorbed by type 8 organisms. After larger quantities of IgG had been absorbed, a prozone phenomenon was apparent. Anticapsular IgG in the absence of complement did not promote phagocytosis by human or rabbit PMNs below levels that caused agglutination. In the presence of complement, only about one tenth as much IgG was necessary to promote phagocytosis. Thus these encapsulated pneumococci can absorb large quantities of specific rabbit IgG antibody of restricted heterogeneity. In the presence of complement, approximately 1% of the antibody that can be absorbed by the organisms will promote phagocytosis. In the absence of complement, approximately 10% of the antibody that can be absorbed is required for agglutination or phagocytosis.