We and other workers have previously reported that u-PA plays a protective role in the antigen-induced arthritis (AIA) model [12] and the mBSA/IL-1 monoarticular arthritis model [13], with u-PA gene-deficient (u-PA-/-) mice developing more severe disease associated with increased intra-articular fibrin deposition. The gene discussed is IL1B; the disease is arthritic joint disease.