The primary objective is to test the efficacy of anti-IL-2R mAb (daclizumab), we propose to treat four groups of patients: 1) moderate aplastic anemia, 2) single lineage failure states including pure red cell aplasia or Diamond Blackfan anemia, 3) relapse of severe aplastic anemia and 4) refractory severe aplastic anemia not responding to both horse and rabbit ATG/CsA. Subjects will receive daclizumab once every other week for a total of 5 doses. Patients relapsing after response to initial treatment may be treated with 2 additional courses of daclizumab. In November 2005, the relapsed and refractory severe aplastic anemia arms were closed by the Data and Safety Monitoring Board (DSMB) for lack of efficacy. In October 2008, the moderate aplastic anemia arm was closed by the DSMB when the data was determined sufficient for making statistical inferences regarding the original hypotheses. In October 2008, accrual to the Diamond Blackfan anemia arm was closed by the DSMB for lack of accrual.