Distinct differences in the capacity of monocyte Fc gamma RII of different individuals to bind or not bind mouse IgG1 defines a polymorphism of Fc gamma RIIa and has previously been defined as the high responder (HR) or low responder (LR) polymorphism of Fc gamma RII. The precise definition of the molecular basis of the human HR/LR polymorphism of Fc gamma RIIa from the peripheral blood mononuclear cells of normal individuals has been determined by anti-CD3 induction of T cell proliferation, the polymerase chain reaction (PCR), nucleotide sequencing, transfection and IgG binding. Amplification of first strand cDNA from mRNA isolated from mononuclear cells was performed by PCR using primers specific for the sequences encoding the leader and cytoplasmic sequences of PCR using primers specific for the sequences encoding the leader and cytoplasmic sequences of Fc gamma RIIa, which is normally expressed in monocytes. Sequencing of the PCR products and transfection of these to Fc gamma R- cells indicated that in Fc gamma RIIa of HR or LR individuals: (i) three nucleotide substitutions (CA to TG and G to A) resulted in the change of glutamine to tryptophan at position 27 (first extracellular domain) and arginine to histidine at position 131 (second extracellular domain); (ii) expression of cDNA encoding the various combinations of these indicated that arginine at position 131 was essential for IgG1 binding whereas the amino acid changes at position 27 had no effect; and (iii) IgG1 at high concentration bound to all allomorphic forms of Fc gamma RIIa.(ABSTRACT TRUNCATED AT 250 WORDS)