Naturally occurring anti-inflammatory factors in the synovial fluids of patients with rheumatic disease and their possible mode of action.

Evidence has been shown that pathological synovial fluid contained a substance capable of stabilizing rat liver lysosomes which was partly inactivated by treatment with trypsin and by storage. Such synovial fluid also appeared to contain a substance which labilized lysosomes and which was more stable than the stabilizing substance. (2) The lysosomal stabilizing substance described above was nondialysable and migrated electrophoretically with the alpha and beta globulins to which class it has been tentatively ascribed. (3) Pathological synovial fluid contained proteases which were active at acid pH and at neutral pH. It also appeared to contain a substance capable of inhibiting these proteases. (4) Alpha2-Macroglobulin has been detected in pathogenic synovial fluid.