Members of the leukocyte immunoglobulin (Ig) G receptor (FcgammaR) family play a key role in antibody-mediated phagocytosis and can either enhance antigen presentation or down-modulate immune responses. We studied immune responses to a pneumococcal conjugate (pneumococcal polysaccharide serotype 1 [PPS1]-tetanus toxoid) and antibody-mediated protection in mice deficient for individual FcgammaRs and complement receptor 3 (CR3). FcR gamma chain-deficient (FcR gamma chain(-/-)) mice, which lack expression of both FcgammaRI and III, had significantly lower anti-PPS1 IgG2b and IgG3 responses than did wild-type mice, whereas FcgammaRII-deficient (FcgammaRII(-/-)) mice had significantly higher IgG2a and IgG3 titers. Wild-type and FcgammaRII(-/-) mice were protected against infection with pneumococcal serotype 1, whereas immunized FcR gamma chain(-/-) and FcgammaRIII-deficient mice were not. Immunized CR3-deficient mice were protected against disease, and complement depletion had little effect on protection. These data indicate that activatory leukocyte FcgammaR, but not FcgammaRII (a murine homologue of human FcgammaRIIb), contributes to IgG-mediated protection against pneumococcal disease.