The pathophysiology of a neutropenic disorder was evaluated in a 60-year-old man using myeloid progenitor cell (CFU-C), colony stimulating activity (CSA), and antineutrophil antibody assays. An immunoglobulin, present in the patient's serum at presentation, was directed against peripheral blood neutrophils, and autologous and allogeneic bone marrow CFU-C. A course of prednisone resulted in resolution of the neutropenia and a disappearance of the cytotoxic antibody. This study suggests that the patient's neutropenic disorder resulted from antibody-mediated destruction of circulating neutrophils and a suboptimal bone marrow granulopoietic response.