A human neutrophil- and eosinophil-specific surface antigen, GFA-2, has been found to be involved in the antibody-dependent cell-mediated cytotoxicity (ADCC) to extracellular targets, and in phagocytosis. The monoclonal antibody (MAb) WEM-G11 was produced which recognizes the GFA-2 structure. This MAb, when used as F(ab')2, stimulated human neutrophils to kill antibody-coated P815 cells and, in the case of human eosinophils, increased their cytotoxic effect on schistosomula of Schistosoma mansoni in a dose-dependent manner. MAb WEM-G11 F(ab')2 also stimulated the phagocytosis of antibody-coated sheep erythrocytes by neutrophils. The effect of WEM-G11 F(ab')2 was specific, because other MAb, whether tested in the form of F(ab')2 fragments or as whole IgG, failed to stimulate neutrophils despite binding to these cells. In contrast to the F(ab')2 fragments of these cells. In contrast to the F(ab')2 fragments of WEM-G11, the whole IgG of this MAb inhibited ADCC and phagocytosis, presumably through interaction with granulocyte Fc receptors. WEM-G11 F(ab')2, and to a greater extent WEM-G11 IgG, induced degranulation, but only from cytochalasin B-treated neutrophils. GFA-2 was absent from lymphocytes, monocytes, erythrocytes, and myeloid and erythroid colony-forming cells, as shown by flow cytometry and colony-forming experiments. GFA-2 appeared at the promyelocytic stage and increased in density as neutrophils became more mature. In the mature neutrophil, the number of binding sites for WEM-G11 were found to be about 20,000 per cell. By immunoprecipitation, it appeared that GFA-2 consisted of a polypeptide chain of about 95,000 m.w. and a low m.w. peptide of about 43,000. By immunoblotting, it was demonstrated that the epitope recognized by WEM-G11 is in the chain of m.w. 95,000. GFA-2 thus constitutes a novel human granulocyte-specific antigen that is central to the functional activity and differentiation of these cells.