Urokinase was administered to knee joint effusions of rheumatoid arthritis (RA) patients who showed no effects to various other RA treatment methods. The fibrinolytic system and the lysosomal enzyme activities of the synovial fluid of 19 RA patients and 10 osteoarthritis (OA) patients were examined before and after treated with urokinase, and the clinical effects of urokinase treatment were examined. 1, Plasmin activities were not present in the synovial fluid of RA patients, but streptokinase activating plasmin (SK-pl.) and euglobulin streptokinase activating plasmin (eug.-SK-pl.) activities were present. Fibrin degradation product (FDP) levels were higher in synovial fluid of RA patients than OA patients. 2, Urokinase was injected into the synovial cavity and at 24 hours after injection, the synovial fluid was collected and the fibrinolytic system was examined. Plasmin activity was not present. Under urokinase treatment, SK-pl. activity was not altered, and in some cases the eug.-SK-pl. activities were elevated compared to untreated synovial fluid. FDP was elevated and activated the fibrinolytic activity. 3, After treatment with urokinase, synovial fluid showed a arise in cathepsin D and β-glucuronidase activities and in synovial cell count. This shows that the synovial inflamation was increased temporarrly by urokinase treatment. 4, Urokinase treatment was not effected in RA cases that showed a low plasminogen and a high plasmin inhibitor in synovial fluid. Joint effusion was not decrease after urokinase treatment in RA cases that showed elevation of plasmin inhibitor and no elevation of FDP levels. 5, Urokinase treatment was effective in about 80% of cases for decreasing RA joint effusion. Urokinase administration resulting in the dissolving of fibrin in RA synovial fluid is a useful treatment.