Several forms of glomerulonephritis are induced by antibodies against self or foreign antigens. Normal B lymphocyte antibody production requires T cell costimulatory signals provided in part by T cell surface expression of gp39/CD40ligand (CD40L) that engages the B cell receptor CD40 and induces B cell differentiation and immunoglobulin class switching. We assessed the effect of disrupting the CD40L-CD40 costimulatory pathway, using a CD40-Ig fusion protein, on the development of membranous glomerulonephritis (MGN) in the mouse. MGN is induced by mouse antibodies that recognize and bind to exogenously administered rabbit anti-mouse renal tubular brush border (RbAMBB) IgG immobilized in the glomerular capillary wall. MGN did not occur in nude mice, showing the need of the T cell function. C57Bl/10 mice immunized with RbAMBB and treated with CD40-Ig fusion protein displayed a delayed autologous response and absence of MGN lesions, while control fusion proteins failed to prevent the development of the disease. These observations provide evidence that disruption of the CD40-CD40L costimulatory pathway can prevent the development of MGN by suppressing T cell-dependent antibody production.