IgG2 antibody is the essential subclass to protect against encapsulated bacteria Fc gamma RIIa is the only Fc gamma receptor that interacts with human IgG2. The two genetically determined allotypes of human Fc gamma RIIa, Fc gamma RIIa-R131 and Fc gamma RIIa-H131 alleles have functionally different reactivities with IgG2 in vitro, and H/H-131 cells have markedly higher binding affinity for human IgG2. Homozygous Fc gamma RIIIb-NA1/NA1 PMNLs show higher phagocytic capacity than Fc gamma RIIIb-NA2/NA2 PMNLs. To evaluate in vivo significance of Fc gamma RIIa and Fc gamma RIIIb allotypes, we analyzed Fc gamma R allotypes in children with bacterial meningitis due to Haemophilus influenzae type b, Streptococcus pneumoniae and Neisseria meningitidis. Fc gamma RII and Fc gamma RIIIb polymorphisms were determined by using quantitative flow cytometry. Fc gamma RIIa were studied in 23 children with bacterial meningitis and 50 healthy Turkish controls, and Fc gamma IIIb in 18 and 43 such individuals, respectively. The distribution of Fc gamma RIIa in the healthy Turkish control group was found to be significantly different from that in the Chinese and Japanese population (p < 0.05), but similar to that of the white population in the USA and the Netherlands. No case (0%) had the Fc gamma RII-H/H-131 Fc gamma RIIIb-NA1/NA1 the corresponding figure in the controls was 4 (9.3%). Homozygous Fc gamma RIIa-H/H-131 phenotype was underrepresented with borderline significance (p: 0.057) in patients below two years of age in comparison with the healthy subjects and with patients with meningitis over two years of age (p: 0.059). Although the study needs to be conducted in a large series of patients in order to draw a firm conclusion, Fc gamma RIIa polymorphism may be a contributing factor to the increased susceptibility to meningitis with encapsulated bacteria in children below two years of age.