BACKGROUND The human Fc gamma receptor IIa (Fc gamma RIIa) is expressed in two polymorphic forms, Fc gamma RIIa-H131 and Fc gamma RIIa-R131, that differ by the replacement of histidine by arginine at position 131. This replacement is caused by a single-nucleotide exchange of A-->G. The resulting receptor forms differ in their binding to human IgG2 and mouse IgG1, which may lead to a different immunologic defense to bacterial polysaccharides and encapsulated bacteria. METHODS A rapid and easy polymerase chain reaction(PCR) method of genotyping the Fc gamma RIIa was developed. Allele-specific primers discriminate between the Fc gamma RIIa-H131 and the Fc gamma RIIa-R131 forms of the receptor. The results were compared with those obtained by another DNA-based genotyping method, in which PCR-amplified DNA was hybridized with allele-specific oligonucleotides, and with a functional phagocytosis assay using mouse IgG1-coated red cells as target antigens. RESULTS The genotypes deduced from the PCR with allele-specific primers were in complete accordance with those obtained by the data from the hybridization of PCR-amplified DNA with allele-specific oligonucleotides. Furthermore, the Fc gamma RIIa genotypes of 28 individuals in all cases corresponded to the functional phenotypes. CONCLUSIONS The use of PCR with allele-specific primers provides a rapid and easily performed method for the determination the Fc gamma RIIa polymorphism.