A rare case of neutropenia in a newborn due to anti-Fc(gamma) RIIIb antibody is described. The newborn, born from the 5th pregnancy, had severe infection and no neutrophils. Full clinical and neutrophil count recovery was observed when the child was 5 weeks old. In maternal serum, panreactive granulocyte alloantibodies were detected. The mother's and her two sisters' granulocytes appeared to be Fc(gamma) RIIIb deficient as found using pheno- and genotyping methods. All of them were healthy. The anti-Fc(gamma) RIIIb specificity of antibodies was identified by the monoclonal antibody immunobilization of neutrophil antigen assay. Such antibodies were not found in both sisters with the Fc(gamma) RIIIb deficiency, although they were pregnant, one of them on the seventh occasion.