Studies in animals and humans with lupus nephritis have demonstrated that essential role of CD40 ligand/CD40 interaction in the production of pathogenic autoantibodies. Anti-CD40L antibody treatment of mice with lupus nephritis ameliorates disease and improves survival. BG9588 is a recombinant humanized monoclonal antibody that specifically binds to CD40 ligand (CD40L) expressed on the surface of activated T lymphocytes and thereby blocks the CD40L/CD40 interaction between T and B cells that is required for the initiation for certain antibody responses. Available preclinical data indicates that BG9588 does not cause a global blockage of the immune response to pathogens and that its effects are reversible upon clearance from the systemic circulation. In this Phase I/II open-label, multiple-dose, multicenter study, 30 subjects with active proliferative lupus nephritis will receive 20 mg/kg BG9588 by IV infusion once every 14 days for three doses, and then once every 28 days for four doses for a total of 7 doses. The primary objective is to determine if BG9588 can decrease proteinuria by greater than or equal to 50 percent from baseline without worsening renal function. The secondary objectives are to determine the safety and pharmacokinetics of BG9588, as well as the effects of BG9588 on renal flares; cellular casts; C(3) levels and anti-dsDNA titers; requirement for prednisone; general lupus activity; and quality of life.