The CH2-CH3 interface of the IgG Fc domain contains the binding sites for a number of Fc receptors including Staphylococcal protein A and the neonatal Fc receptor (FcRn). It has recently been proposed that the CH2-CH3 interface also contains the principal binding site for an isoform of the low affinity IgG Fc receptor II (FcγRIIb). The FcγRI and FcγRII binding sites have previously been mapped to the lower hinge and the adjacent surface of the CH2 domain although contributions of the CH2-CH3 interface to binding have been suggested. This study addresses the question whether the CH2-CH3 interface plays a role in the interaction of IgG with FcγRI and FcγRIIa. We demonstrate that recombinant soluble murine FcγRI and human FcγRIIa did not compete with protein A and FcRn for binding to IgG, and that the CH2-CH3 interface therefore appears not to be involved in FcγRI and FcγRIIa binding. The importance of the lower hinge was confirmed by introducing mutations in the proposed binding site (LL234, 235AA) which abrogated binding of recombinant soluble FcγRIIa to human IgG1. We conclude that the lower hinge and the adjacent region of the CH2 domain of IgG Fc is critical for the interaction between FcγRIIa and human IgG, whereas contributions of the CH2-CH3 interface appear to be insignificant.