Recently, a new alloantigen on IgG Fc receptor type IIIb (Fc gamma RIIIb), SH, was described (Bux et al, Blood 89:1027, 1997). We identified three healthy individuals whose neutrophils reacted positively with the SH antiserum. The neutrophil antigen (NA) phenotype of all three donors was NA(1+,2+). Analysis of genomic DNA showed that the three donors were positive for the described SH-encoding mutation in the NA2-Fc gamma RIIIB gene, 266C-->A. However, NA(1,2) genotyping and nucleotide sequencing of an NA2-specific fragment amplified from the genomic DNA fragment showed that these individuals carried three Fc gamma RIIIB genes, namely, NA1-Fc gamma RIIIB, NA2-Fc gamma RIIIB, and SH-Fc gamma RIIIB, encoding NA1-Fc gamma RIIIb, NA2-Fc gamma RIIIb, and SH-Fc gamma RIIIb, respectively. Southern blot analysis confirmed these findings. Furthermore, all three transcripts were isolated from neutrophil mRNA. To investigate whether the presence of three Fc gamma RIIIB genes resulted in a higher membrane expression of Fc gamma RIIIb, we measured the reactivity of neutrophils from NA(1+,2+)SH(+) individuals with a panel of CD16 monoclonal antibodies (MoAbs) in comparison with neutrophils from NA(1+,2+)SH(-) controls. Reactivity of four different anti-pan-Fc gamma RIII MoAbs and NA2-specific MoAb GRM1 was higher with SH(+) neutrophils compared with controls, whereas that of NA1-specific MoAbs was similar, which is in concordance with the results from the genomic analysis. We observed that reactivity with NA2-specific CD16 MoAb PEN1 was sixfold higher in SH(+) individuals compared with controls. Apparently, the 60Ala-->Asp substitution in SH-Fc gamma RIIIb influences the epitope recognized by PEN1. In conclusion, we identified three NA(1+,2+)SH(+) individuals carrying three Fc gamma RIIIB genes and observed a clear gene-dosage effect on the level of expression of neutrophil Fc gamma RIIIb.