Heterogeneity of synovial fluid fibronectin was studied by means of Laurell cross-immunoelectrophoresis in patients with rheumatoid arthritis, posttraumatic synovitis and other arthropathies. Prior to hyaluronidase treatment all the synovial fluid samples exhibited the fibronectin heterogeneity, which disappeared after the action of hyaluronidase. The data obtained suggest that complexes of fibronectin and hyaluronic acid are responsible for physico-chemical heterogeneity of fibronectin in synovial fluid.