The low-affinity receptor for IgG, Fc gamma RII, and the high-affinity receptor for IgE, Fc epsilon RI, are functionally distinct but structurally homologous receptors. These characteristics have been exploited using a chimeric receptor strategy to examine segments of human Fc gamma RII for IgG-binding function. A series of chimeric receptors was generated by exchanging coding regions of the extracellular ligand-binding regions between Fc gamma RII and the Fc epsilon RI alpha chain using splice overlap extension by the polymerase chain reaction. The expression of these chimeric receptors in COS-7 cells and analysis of their IgG/IgE binding capacities have enabled the Ig-binding region of Fc gamma RII to be localized to a subregion of the second extracellular domain. The localization of the Ig-binding region of Fc gamma RII has provided the opportunity of performing site-directed mutagenesis to determine the key amino acids involved in the interaction of the receptor with IgG. These findings demonstrate that the chimeric receptor approach is a powerful technique for the dissection of structure/function relationships of structurally related yet functionally different molecules.