Natural killer (NK) cells are important in host defense against viruses and tumors and can induce death of virally infected cells following engagement of cell surface receptors. Human NK cells express receptors for the Fc portion of IgG which stimulate antibody-dependent cell-mediated cytotoxicity and induce cytokine production. We have shown that NK cells from certain individuals can express, in addition to CD16 (FcgammaRIIIa), isoforms of CD32 (FcgammaRIIc1-4). Expression of CD32 on NK cells is dependent on an allelic polymorphism of the FcgammaRIIC gene. We analyzed the expression and function of CD32 on NK cells from 31 normal donors. Fourteen of the 31 (45%) donors expressed CD32 on their NK cells. Molecular characterization of FcgammaRIIc isoforms expressed by the CD32+ donors revealed that the majority of donors expressed the FcgammaRIIc1 isoform. Interestingly, 3 of the 14 positive donors did not express FcgammaRIIc1, and we identified a novel isoform, FcgammaRIIc5, expressed by these individuals. The expression of this isoform was correlated to a second allelic polymorphism that controls exon splicing. One of the three was found to express FcgammaRIIb on the NK cells. Biochemical analysis revealed that CD32+ donors of both types expressed a 40-kDa protein, specifically immunoprecipitated by anti-CD32 monoclonal antibodies. Functionally, only individuals expressing the FcgammaRIIc1 isoform were able to trigger reverse antibody-dependent cell-mediated cytotoxicity via CD32 whereas a CD32+ individual expressing the FcgammaRIIb isoform was unable to trigger this function. These results demonstrate that the presence of multiple allelic polymorphisms in the FcgammaRIIC gene determine the expression and function of CD32 on NK cells.