A child with congenital neutropenia was studied using bone marrow culture and ultrastructural and cytochemical techniques. The patient's marrow cells formed a large number of granulocytic colonies of normal size in culture, and her peripheral blood leukocytes produced adequate colony-stimulating factor. No serum inhibitors were identified. The patient's promyelocytes from direct marrow and culture appeared normal in ultrastructure, and primary granules, contained peroxidase and acid phosphatase activity. Myelocytes and rare segmented neutrophils from direct marrow specimens demonstrated atypical notched nuclei, myelin figures in Golgi lamellae and primary (azurophilic) granules, and no identifiable secondary (specific) granules. These data indicate an intrinsic neutrophil defect which allows normal proliferation of precursor cells, but results in abnormal granulogenesis and apparent inability to form secondary granules.