In vitro coculture studies of normal T lymphocytes, natural killer (NK) cells, or monocytes with human hematopoietic progenitor cells (HPC) suggest a potential role for immune cells in the regulation of hematopoiesis. In some patients with severe aplastic anemia or selective pure red cell, white cell, or megakaryocyte aplasia, activated T cells, NK cells, or monocytes mediate marrow failure by suppressing growth of HPC. Immunosuppressive agents such as corticosteroids, antithymocyte globulin, cyclophosphamide, or cyclosporin A may dramatically improve blood cell production in patients with marrow failure by removing or modulating immune suppressor cells.