Human natural killer (NK) cells were thought to express only FcγRIIIA (CD16), but recent reports have indicated that NK cells also express a second type of FcγR, ie, FcγRII (CD32). We have isolated, cloned, and sequenced full-length cDNAs of FcγRII from NK cells derived from several normal individuals that may represent four different products of the FcγRIIC gene. One transcript (IIc1) is identical with the already described FcγRIIc form. The other three (IIc2-IIc4) appear to represent unique, alternatively spliced products of the same gene, and include a possible soluble form. Analyses of the full-length clones have revealed an allelic polymorphism in the first extracellular exon, resulting in either a functional open reading frame isoform or a null allele. Stable transfection experiments enabled us to determine a unique binding pattern of anti-CD32 monoclonal antibodies to FcγRIIc. Further analyses of NK-cell preparations revealed heterogeneity in CD32 expression, ranging from donors lacking CD32 expression to donors expressing high levels of CD32 that were capable of triggering cytotoxicity. Differences in expression were correlated with the presence or absence of null alleles. These data show that certain individuals express high levels of functional FcγRIIc isoforms on their NK cells.