To identify the relationship of the severity of inflammation and fibrinolytic activity in arthritis, the fibrinolytic activity of synovial fluid was studied in acute experimental arthritis induced by injecting monosodium urate crystals into dogs' knee joints. The maximum activity in the synovial fluid was observed 6 h after crystal injection. It was inferred that the fibrinolytic activity was mainly due to plasminogen activator based on fibrin plate assays, substrate specificity, inhibitor effects and zymography. On the other hand, the activity of lysosomal enzymes (beta-glucuronidase and cathepsin G) reached a peak in the synovia after 12 h. Histological examination of the synovial membrane after 12 h also showed greater inflammation than at 6 h. The peak in fibrinolytic activity preceded the peak of lysosomal enzymes and histological changes. These results suggest that an increase in fibrinolytic activity by plasminogen activator may contribute to the development of an acute inflammatory response.