CHRONIC idiopathic neutropenia is characterized by maturational arrest of neutrophil precursors in the bone marrow and a circulating neutrophil count of less than 1500 cells per microliter. Other hematopoietic cell counts are usually normal, and antineutrophil antibodies are absent. Clinically, patients have mucosal ulcerations, periodontal disease, and intermittent infections. Symptomatic or prophylactic treatment with antibiotics has been the mainstay of therapy. The cause of this neutropenia is unknown. Although early studies suggested that colony-stimulating activity in the urine and serum of patients was similar to that found in normal persons,1 the production of specific colony-stimulating factors has not been evaluated.