Two polymorphic forms of Fc receptor III (FcR III) are expressed on human neutrophils. These differ with respect to their apparent molecular masses after digestion with N-glycanase, and with respect to their reactivity with MAb Gran 11 and alloantisera which recognize determinants (NA1 and NA2) of the biallelic neutrophil antigen (NA) system. To determine the molecular basis for this polymorphism we isolated RNA from neutrophils of NA1NA1 and NA2NA2 homozygotes and synthesized corresponding cDNAs. cDNAs encoding FcR III were then amplified using the polymerase chain reaction, cloned, and sequenced. The cDNA that encodes FcR III on NA1NA1 neutrophils differed from the cDNA that encodes FcR III on NA2NA2 neutrophils at five nucleotides, predicting four amino acid substitutions. As a result, NA1 FcR III has only four potential N-linked glycosylation sites as compared with six in NA2 FcR III. The amino acid substitutions and differences in the number of potential N-linked glycosylation sites probably account for the different forms of neutrophil FcR III observed after digestion with N-glycanase and for the antigenic heterogeneity of this receptor.