The immune response depends on the binding of opsonized antigens to cellular Fc receptors and the subsequent initiation of various cellular effector functions of the immune system. Here we describe the crystal structures of a soluble Fcγ receptor (sFcγRIII, CD16), an Fc fragment from human IgG1 (hFc1) and their complex. In the 1:1 complex the receptor binds to the two halves of the Fc fragment in contact with residues of the Cγ2 domains and the hinge region. Upon complex formation the angle between the two sFcγRIII domains increases significantly and the Fc fragment opens asymmetrically. The high degree of amino acid conservation between sFCγRIII and other Fc receptors, and similarly between hFc1 and related immunoglobulins, suggest similar structures and modes of association. Thus the described structure is a model for immune complex recognition and helps to explain the vastly differing affinities of other FcγR–IgG complexes and the FcεRIα–IgE complex.