For the purpose to clarify the role played by plasmin, a proteolytic enzyme, in rheumatoid arthritis, the fibrinolytic activities of the sera and synovial fluids from rheumatoid arthritic patients were studied. The results are briefly presented as follows:
1. In rheumatoid arthritis it was found that the fibrinolytic activity was higher in the serum and synovial membrane than that in normal individuals.
2. The fibrinolytic activies in the group of patients showing the serum and synovial fluid positive to CRP reaction were more marked than those in the negatiye group.
3. In the active stage of rheumatic activity the serum fibrinolysis was also more marked than that at the rest stage. In the observations carried out for a long period of time on individual cases the extent of the serum fibrinolysis coincided well with the rheumatic activity.
4. There was a correlation between the fibrinolysis of synovial fluid and the quantity of fluid accumulated.
5. The fibrinolytic activities of the synovial fluid obtained from the knee joint previously injected with osmic acid paralleled well with the inflammatory symptoms, and also parallaled with the amounts of fluid accumulated as well as with the synovial response to CRP reaction and the cell counts.
6. In those of coating type where fibrin exudate on the synovial membrane was copious histopathologically, they also showed high fibrinolytic activity of synovial fluid.