Matrix metalloproteinase (MMP)9/gelatinase B is increased in various nephropathies. To investigate its role, we used a genetic approach. Adult MMP9-deficient (MMP9^−/−) mice showed normal renal histology and function at 3 mo. We investigated the susceptibility of 3-mo-old mice to the accelerated model of anti-glomerular basement membrane nephritis, in which fibrin is an important mediator of glomerular injury and renal impairment. Unexpectedly, nephritis was more severe in MMP9^−/− than in control mice, as attested by levels of serum creatinine and albuminuria, and the extent of crescents and fibrin deposits. Circulating or deposited immunoglobulin G, interleukin (IL)-1β, or IL-10 were the same in MMP9^−/− and MMP9^+/+ mice. However, we found that fibrin is a critical substrate for MMP9, and in its absence fibrin accumulated in the glomeruli. These data indicate that MMP9 is required for a novel protective effect on the development of fibrin-induced glomerular lesions.