Murine phagocytes express three different activating IgG FcgammaR: FcgammaRI is specific for IgG2a; FcgammaRIII for IgG1, IgG2a, and IgG2b; and FcgammaRIV for IgG2a and IgG2b. Although the role of FcgammaRIII in IgG1 and IgG2a anti-RBC-induced autoimmune hemolytic anemia (AIHA) is well documented, the contribution of FcgammaRI and FcgammaRIV to the development of IgG2a- and IgG2b-induced anemia has not yet been defined. In the present study, using mice deficient in FcgammaRI, FcgammaRIII, and C3, in combination with an FcgammaRIV-blocking mAb, we assessed the respective roles of these three FcgammaR in the development of mild and severe AIHA induced by two different doses (50 and 200 microg) of the IgG2a and IgG2b subclasses of the 34-3C anti-RBC monoclonal autoantibody. We observed that the development of mild anemia induced by a low dose of 34-3C IgG2a autoantibody was highly dependent on FcgammaRIII, while FcgammaRI and FcgammaRIV additionally contributed to the development of severe anemia induced by a high dose of this subclass. In contrast, the development of both mild and severe anemia induced by 34-3C IgG2b was dependent on FcgammaRIII and FcgammaRIV. Our results indicate differential roles of the three activating FcgammaR in IgG2a- and IgG2b-mediated AIHA.