The Fc receptors (FcR), which belong to the immunoglobulin (Ig) superfamily, bind to specific Ig isotypes with varying affinities triggering complex immune defense responses. Several of the FcR that lack signaling motifs in their cytoplasmic domains rely on associated subunits to transmit signals. Two classes of FcR that bind the Fc portion of IgG, Fc gamma RI, and Fc gamma RIIIa associate with a subunit shared among several FcR, the gamma chain, which is involved in receptor expression and signal transduction. In this report, we propose that a novel role for gamma chain is to enhance the affinity of Fc gamma R for ligand. Our findings demonstrate that Fc gamma RI requires gamma -chain association to attain high affinity binding for monomeric IgG, and suggest that the intermediate binding affinity of the Fc gamma RIIIa isoform results from its association with gamma chain. The affinity increase conferred by gamma chain appears to be mediated through the transmembrane domain of the Fc gamma R, with no requirement for the cytoplasmic domain of the receptor.