Maternal genomic neutrophil FcRIII deficiency leading to neonatal isoimmune neutropenia. The healthy mother of a child with transient immune neutropenia was found to be "NA-null." The mother's neutrophils did not react with anti-NA1 and anti-NA2 antibodies (polyclonal human alloantibodies and mouse monoclonal antibodies). A healthy donor was discovered during routine neutrophil antigen typing whose neutrophils were also "NA-null." This NA-phenotype was due to the absence of FcRIII (CD16 antigen) on neutrophils as demonstrated with anti-FcRIII monoclonal antibodies. The neutrophils of these two individuals were not able to bind dimeric immunoglobulin G. However, their cells had a normal expression of other phosphatidylinositol (PI)-linked membrane glycoprotein (CD24, CD67, and CLB gran/5 antigens), ruling out the existence of a PI-linkage defect, such as paroxysmal nocturnal hemoglobinuria. The mother (propsitus) had isoantibodies in her blood against neutrophil-FcRIII without allospecificity, apparently produced during pregnancy and responsible for the neutropenia of her child. The expression of FcRIII on natural killer lymphocytes of both individuals was normal. FcRIII is encoded by two separate genes, one (FcRIII-1) for the neutrophil-PI-linked receptor, another (FcRIII-2) for the natural killer cell and macrophage-transmembrane receptor. By messenger RNA and DNA analysis (with an FcRIII-cDNA probe and restriction endonucleases) the neutrophil-FcRIII deficiency appeared to be due to deletion of the FcRIII-1 gene in both individuals, while the FcRIII-2 gene was normally present. The parents of the propositus were found to be heterozygous for this defect. Thus, FcRIII-1 gene deficiency of the mother may be a cause of (iso)immune neutropenia of the newborn. Whether this deficiency may have other clinical consequences has to be studied. 