The present report describes the natural history of an experimental acute glomerulonephritis with massive proteinuria induced by a single intravenous injection of a (mouse) monoclonal anti-rat Thy 1.1 antibody into the rat. The disease is characterized by direct although transient binding of this monoclonal antibody to glomerular basement membrane and mesangium after injection as demonstrated by immunofluorescence microscopy. Immediate activation of complement occurs as shown by glomerular deposition of C3 and C9. Concomitant activation of the coagulation cascade is reflected by the presence of fibrinogen deposits in the affected glomeruli. One hour after injection mesangial alterations are prominent including condensation of mesangial cell chromatin, and lysis of mesangial cells as shown by light- and electron- microscopy, leading to the formation of aneurysms in the capillary tuft. Commencing on day 4 mesangial cell proliferation can be observed, accompanied by glomerular crescent formation at day 14, which decreases gradually 3 weeks after antibody administration, whereas mesangial hypercellularity can be observed up week 10 after intravenous injection of the antibody. The disease is clinically characterized by a massive transient proteinuria starting immediately after antibody injection, reaching mean values of 300 mg/24 hours at days 2 to 4, gradually decreasing to normal levels after 3 weeks. It is concluded that in this unique model of glomerulonephritis induced by a monoclonal antibody, recognition of Thy 1.1 epitopes as well as activation of complement including the C5-C9 membrane attack complex may play a major role in the pathogenesis of this experimental disease.