Rituximab to Treat Moderate Aplastic Anemia, Pure Red Cell Aplasia, or Diamond Blackfan Anemia

* INCLUSION CRITERIA: Diagnosis of acquired moderate aplastic anemia defined as aplastic anemia (hypocellular bone marrow) and no evidence for an underlying disease process and depression of at least two out of three blood counts below these values: * Absolute neutrophil count (ANC) equal to or less than l200/mm(3) * platelet count equal to or less than 70,000/mm(3) * anemia with hemoglobin equal to or less than 8.5 g/dl or absolute reticulocyte count equal to or less than 60,000/mm(3) in transfusion-dependent patients but not fulfilling the criteria for severe disease defined by bone marrow cellularity less than 30% (excluding lymphocytes) and depression of at least two of the three peripheral counts: * ANC equal to or less than 500/ul * platelet count equal to or less than 20,000/ul * reticulocyte count less than 60,000/ul Or Diagnosis of pure red cell aplasia or Diamond Blackfan anemia requiring red blood cell (RBC) transfusions Pure red cell aplasia is defined by * anemia, * reticulocytopenia (reticulocyte count equal to or less than 50,000/ mm(3)) * and absent or decreased marrow erythroid precursors Diamond Blackfan anemia is defined by * anemia, * reticulocytopenia (reticulocyte count equal to or less than 50,000/ mm(3)) * and absent or decreased marrow erythroid precursors diagnosed at an early age Because this population is prone to dry bone marrow aspirates, subjects from whom sufficient bone marrow cannot be collected for the evaluation of cellularity will not be excluded provided they meet all other inclusion criteria based on peripheral blood counts. Pure Red cell Aplasia and Diamond Blackfan patients must be age greater than or equal to 2 years old and weight greater than 12 kg; Moderate Aplastic anemia patients must be age greater than or equal to 18. Refractory to at least 1 course of immunosuppressive therapy or relapsed disease after prior immunosuppressive therapy (PRCA/DBA patients only). Patients or their parent(s)/responsible guardian(s) must be able to comprehend and be willing to sign an informed consent. EXCLUSION CRITERIA: Current diagnosis of Fanconi's anemia or other congenital bone marrow failure syndromes except for DBA History of a cytogenetic abnormality indicating myelodysplasia (MDS) Active infection not adequately responding to appropriate therapy HIV positivity Positive anti- hepatitis B core antibody (antiHBc) or HBsAG History of clinically significant arrhythmia Known anaphylaxis or immunoglobulin E (IgE) mediated hypersensitivity to murine proteins or to any component of this product. Moribund status or concurrent hepatic, renal, cardiac, neurologic, pulmonary, infectious, or metabolic disease of such severity that it would preclude the patient's ability to tolerate protocol therapy, or that death within the next month is likely Potential subjects with cancer who are on active chemotherapeutic treatment or who take drugs with hematological effects will not be eligible. History of recent or ongoing B19 parvovirus infection Psychiatric, affective, or other disorder that may compromise the ability to give informed consent or to cooperate in a research study. Pregnancy or lactation or unwillingness to take contraceptives Participation in any other investigational drug trial or exposure to other investigational agents (other than hematopoietic growth factors) within 30 days of study entry. Use of low dose immunosuppressive agents may continue at the PIs discretion provided that the patient has been taking this drug for at least 3 months.