Membranoproliferative glomerulonephritis (MPGN) is a relatively-rare, immune-mediated kidney disease. All current therapies are inadequate and MPGN frequently leads to kidney failure. This study is a 10 patient trial of the monoclonal antibody rituximab for adult patients with MPGN. Study patients will receive 2 doses of rituximab intravenously on days 1 and 15 and will then be followed for 1 year.