Isolated and washed cryoglobulins obtained from 30 patients with different diseases were studied for the presence of fibronectin by immunoelectrophoresis. Two of the isolated cryoglobulins were of the type I, 3 were of the type II, while the other 25 were of type III. Despite extensive washing, fibronectin was detected in all the cryoprecipitates. Our results indicate the constant presence of fibronectin in the cryoprecipitates from patients with different diseases and types of cryoglobulin. The possible mechanisms by which fibronectin could act in the precipitation of immunoglobulins are discussed.