A black patient with severe aplastic anemia is described who underwent successful bone marrow transplantation from a sibling with chronic neutropenia. During an evaluation to identify a suitable donor, it was found that the majority of family members tested had neutropenia, with no familial history of significant infections or related hospitalizations. In vitro hemopoietic culture studies of marrow from the patient's HLA-MLC-matched siblings showed normal numbers of pluripotential and committed hemopoietic progenitors; in vitro hemopoietic colony formation from the patient was markedly subnormal, consistent with the clinical picture of severe aplastic anemia. Following appropriate conditioning therapy, marrow transplanted from one of these neutropenic sibs produced full hematopoietic reconstitution. Posttransplant marrow culture studies of the patient showed restoration of a normal pattern of in vitro hemopoiesis. The in vitro culture studies and clinical experience in this patient support the concept that chronic neutropenia of blacks is not primarily a marrow progenitor cell disorder but, more likely, a manifestation of a genetically determined alteration in granulocyte kinetics.