The plasminogen activators, urokinase PA (u-PA) and tissue-type PA (t-PA), are believed to play important roles in inflammatory cell infiltration, fibrin deposition, and joint destruction associated with rheumatoid arthritis; however, their precise roles in such processes, particularly u-PA, have yet to be defined. Using gene-deficient mice we examined the relative contribution of the PAs to the chronic systemic collagen-induced arthritis model. Based on clinical and histological assessments, u-PA−/− mice developed significantly milder disease and t-PA−/− mice more severe disease compared with the relevant wild-type mice. Fibrin deposition within joints paralleled disease severity and was particularly pronounced in t-PA−/− mice. Likewise, cytokine levels in the synovium reflected the severity of disease, with interleukin-1β levels in particular being lower in u-PA−/− mice and increased in t-PA−/− mice. The antibody response to type II collagen was normal in both knockouts; however, T cells from u-PA−/− mice had a reduced proliferative response and produced less interferon-γ on antigen stimulation in vitro. These results indicate that the major effect of u-PA in the collagen-induced arthritis model is deleterious, whereas that of t-PA is protective. Our data highlight the complexities of PA function, and suggest that approaches either to target u-PA or to enhance local t-PA activity in joints may be of therapeutic benefit in rheumatoid arthritis.