The functionally active IgG subclasses and the portion of the antibody molecule mediating opsonophagocytosis of type III, group B Streptococcus (III-GBS) have not previously been investigated. Serum from a healthy adult immunized with III-GBS vaccine was subjected to ion exchange chromatography, yielding 2,950 mg of IgG/dl and 1,440 micrograms of III-GBS-specific antibody/ml. Papain digestion for selective cleavage of IgG1 and IgG3 yielded an IgG2-rich pool that contained 31% of the initial IgG but 69% of the antibody to III-GBS. This pool retained opsonophagocytic activity of antibody to III-GBS at levels similar to that for the initial IgG pool. Analysis of F(ab')2 fragments of pepsin-digested IgG revealed that the level of alternative pathway-mediated opsonophagocytosis of III-GBS was similar to that mediating functional activity in the initial IgG preparation. Therefore, IgG2 is an active opsonin for III-GBS, but divalency appears to be required for alternative pathway-mediated opsonophagocytosis.