Membranoproliferative glomerulonephritis (MPGN) is a relatively-rare, immune-mediated glomerular disease. There is no accepted therapy and all current therapies are inadequate. Current therapeutic options include immunosuppression with corticosteroids alone or in combination with alkylating agents, antiplatelet therapy with aspirin and/or dipyridamole and/or warfarin, and angiotensin converting enzyme inhibitors and/or angiotensin receptor blockers. As with other glomerular diseases the amount of protein in the urine correlates well with the long-term prognosis. Thus, this parameter has been used in previous studies, and will be used in this study, as the primary indicator of therapeutic efficacy. We propose a pilot study to test the hypothesis that selective B lymphocyte depletion will result in disappearance of pathogenic antibodies and induce remission of proteinuria in patients with idiopathic membranoproliferative glomerulonephritis. Our population will be 10 adults with MPGN involving either the native kidneys or a renal transplant. We will enroll patients with a glomerular filtration rate (GFR) greater than or equal to 25 ml/min, as estimated by creatinine clearance, and with a 24 hour urinary ratio of protein to creatinine greater than or equal to 1, while receiving an angiotensin converting enzyme inhibitor (ACEI) or angiotensin II receptor blocker (ARB). Patients will receive Rituximab 1g on Day 1 and 15. Patients will be followed for 1 year following completion of treatment. The primary outcome will be the change in urinary protein excretion at 6 months. Secondary outcomes will include changes in the GFR, changes in urinary protein excretion at 3, 9, and 12 months, the rate of change in urinary protein excretion, serum albumin concentration, serum cholesterol, the number of complete and partial remissions, time to remission, and the number of relapses.