We have genotyped 53 individuals from three ethnic groups (Japanese, Chinese, Asian Indian) for an allotypic polymorphism of a widely expressed low affinity Fc receptor for IgG (Fc gamma RIIA). The method, requiring PCR amplification of genomic DNA and Southern analysis with allele specific oligonucleotide probes, detects a single nucleotide difference (G or A) at base 494 which results in an arginine (R) or histidine (H) at amino acid 131 of the Fc gamma RIIA protein. This polymorphism has been shown to determine the affinity of the receptor for hIgG2; Fc gamma RIIA-H131 has a high affinity for hIgG2, while Fc gamma RIIA-R131 binds hIgG2 weakly. We found that the Japanese and Chinese groups have an increased frequency of the H/H131 allotype (61 and 50% respectively) as compared to the Caucasian group (23%), in agreement with previously reported phenotype data. The genotype distribution of the Asian Indian group was not different from our Caucasian group. The shifts in frequency of the R131 and H131 alleles in different populations may have implications for disease susceptibility when the hIgG2 antibody isotype predominates.