Participants receive four doses of rituximab, once a week for 4 weeks through a needle in an arm vein. The infusion rate depends on how well the patient tolerates the drug. The first infusion usually takes 4 to 6 hours and the rest take 3 to 4 hours. The first and fourth infusions are given at NIH; the second and third may be given at NIH or by a patient's referring doctor. Patients who respond to rituximab but then relapse may receive one additional course of four doses. Patients may continue with transfusions and their current medications, including growth factors (e.g., Epogen and Neupogen) while on study, but may have to stop taking immunosuppressive drugs, such as prednisone or cyclosporine. Patients who must start another immunosuppressive medication are taken off rituximab and followed for safety with clinic visits one week and then once a month for 6 months after the first dose of rituximab.