The low-affinity human Fc gamma RIIa is encoded by a single gene with allelic variation, defined by low-responder and high-responder alleles (LR and HR). The HR Fc gamma RIIa transcript interacts strongly with murine (m) IgG1 complexes, in contrast to the LR Fc gamma RIIa. Furthermore, the transcripts can be discriminated by mAb 41H16, which recognizes an epitope expressed on the HR Fc gamma RIIa molecule. We report that this receptor is also polymorphic in its reactivity with human (h) IgG2. Binding studies using well-defined hIgG dimers revealed that LR Fc gamma RIIa molecules can efficiently bind hIgG2, in contrast to HR Fc gamma RIIa. Previous work of others showed one amino acid difference between the allelic forms of Fc gamma RII. We, however, found a second amino acid difference between both allelic forms. In this study, hybrid Fc gamma RIIa molecules were constructed to determine the epitope for mAb 41H16 and the binding domain for mIgG1 and hIgG2 complexes. Our data point to the importance of the amino acid at position 131, located in the second Ig-like domain of Fc gamma RIIa. When an arginine residue is present at amino acid position 131, the receptor is recognized by mAb 41H16. Furthermore, the receptor can bind mIgG1-sensitized indicator E, but binds hIgG2 dimers only weakly. When a histidine residue is present at this amino acid position, hIgG2 dimers do bind efficiently to Fc gamma RII, whereas mIgG1-sensitized E and mAb 41H16 exhibit a strongly diminished binding.