Many bone marrow failure syndromes in humans are now recognized to result from immunological mechanisms. These diseases include aplastic anemia; single hematopoietic lineage failures such as pure red cell aplasia, Diamond Blackfan Anemia, agranulocytosis, and amegakaryocytic thrombocytopenic purpura; and some types of myelodysplasia. Patients with these conditions, who may suffer variable degrees of anemia, leukocytopenia, and thrombocytopenia, alone or combination, have been shown to respond to a wide variety of immunosuppressive agents, ranging from corticosteroids to cyclosporine (CSA) and antithymocyte globulin (ATG). However, except for severe aplastic anemia, which has been shown to be appropriately treated with either bone marrow transplantation or a combination of ATG and CSA, single agents or regimens have usually not been applied systematically to other immune-mediated hematologic diseases. In an effort to discover other and especially less toxic treatments for immunologically-mediated bone marrow diseases, we seek to apply a new therapy, a humanized anti-interleukin-2 receptor (lL-2R) monoclonal antibody (mAb ), to a subset of patients with bone marrow failure. Anti-IL-2R mAb acts against activated lymphocytes, thus sharing an important mechanism of action with ATG. However, the mAb is much less toxic than ATG and may be administered to outpatients at relatively infrequent intervals (every 2 weeks).