Fcbold gamma receptors bind IgG to initiate cellular responses against pathogens and soluble antigens. We have determined the three−dimensional structure of the extracellular portion of human Fcbold gammaRIIa to 2.0 Å resolution providing a structural basis for the unique functions of the leukocyte FcR family. The receptor is composed of two immunoglobulin domains and arranged to expose the ligand−binding site at one end of domain 2. Using alanine mutants we find that the binding sites for IgG1 and 2 are similar but the relative importance of specific regions on the receptor varies. In crystals, Fcbold gammaRIIa molecules associate to resemble VLVH dimers, suggesting that two Fcbold gammaRIIa molecules could cooperate to bind IgG in an asymmetric manner.