Murine phagocytes express three different activating IgG FcγR: FcγRI is specific for IgG2a; FcγRIII for IgG1, IgG2a, and IgG2b; and FcγRIV for IgG2a and IgG2b. Although the role of FcγRIII in IgG1 and IgG2a anti-RBC-induced autoimmune hemolytic anemia (AIHA) is well documented, the contribution of FcγRI and FcγRIV to the development of IgG2a-and IgG2b-induced anemia has not yet been defined. In the present study, using mice deficient in FcγRI, FcγRIII, and C3, in combination with an FcγRIV-blocking mAb, we assessed the respective roles of these three FcγR in the development of mild and severe AIHA induced by two different doses (50 and 200 μg) 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 FcγRIII, while FcγRI and FcγRIV 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 FcγRIII and FcγRIV. Our results indicate differential roles of the three activating FcγR in IgG2a-and IgG2b-mediated AIHA.