Biologic properties of antibodies are known to be mediated by the Fc portion of the H chains. In the present study such properties as complement fixation, cutaneous anaphylaxis, and macrophage cytophilia were examined in relation to the CH2 and CH3 domains of rabbit IgG. Fragments containing but one of these domains were prepared from plasmin and papain digests. Facb fragments of anti-DNP antibodies, together with the antigen DNP-BSA, were able to fix complement by the classical pathway, a result which implicates the CH2 domain; however, guinea pig Fab fragments directed to regions of the rabbit antibody molecule other than CH2 were able to inhibit complement fixation. Facb fragments were unable to mediate PCA or reverse PCA reactions in guinea pigs, nor were CH3 fragments active in tests of reverse PCA or inhibition of PCA. These results suggest that the entire Fc region is needed for cutaneous anaphylaxis. The ability to bind to guinea pig lung macrophages was studied with a rosette technique. Facb fragments were active whereas CH3 fragments failed to inhibit. It is suggested that although some effector functions of antibodies can be assigned to individual domains, others require the entire Fc region.