In 20 consecutive rheumatoid arthritis patients, 14 women and 6 men, age 26--76, average 62 years, the concentration of the recently found "primary plasmin inhibitor" and phase proteins was estimated in plasma and synovial fluid. In 12 patients a complex between the inhibitor and plasmin could be demonstrated by crossed immunoelectrophoresis into immunoglobulins against the primary plasmin inhibitor and immunoglobulin against plasminogen. Only free inhibitor was found in corresponding plasma. All plasminogen present in synovial fluid could be activated to plasmin upon addition of urokinase (24 nM/1). In those patients where enzyme-inhibitor complex in synovial fluid was present, a higher concentration of phase proteins in synovial fluid was found, indicating an increased degree of inflammation despite identical scores in the Lansbury clinical index in the two groups. From these experiments it was concluded that the fibrinolytic capacity in rheumatoid synovial fluid is not decreased. It is suggested that the fibrin-like material in synovial tissue and upon the synovial membrane is a poor substrate for plasmin.