Rituximab is a human/murine chimeric monoclonal anti CD-20 antibody that is extensively used in patients with B-cell non-Hodgkin's lymphoma, or with autoimmune disease. The incidental observation of improvement in aGVHD following rituximab infusion for transplant-associated thrombotic thrombocytopenic purpura (TA-TTP) and subsequent complete resolution of multi-agent refractory aGVHD in two patients forms the basis of this proposed pilot study. Table-2 (in full protocol) describes the patients and their responses to rituximab at our own institution.