Fibronectin of human plasma associated readily with denatured collagen but gave only a weak reaction with the native protein. In the presence of heparin, however, solutions of native collagen type III, and fibronectin produced precipitates at an ionic strength of 0.2. In the presence of fibronectin and optimal additions of heparin, up to 60% of soluble native 125I-collagen type III, but only about 10% of native 125I-collagen type I, were insolubilized. Heparin also enhanced the formation of insoluble complexes from fibronectin and denatured collagen type I and type III. In the absence of collagen 125I-fibronectin was partially precipitated by heparin. Electron micrographs showed filamentous structures. Collagen did not increase the amount of 125I-fibronectin precipitated by heparin unless a critical collagen concentration was exceeded. It is suggested that heparin induced the transition of fibronectin from a globular to an elongated form, capable of forming filamentous precipitates which adsorb native collagen. Hyaluronic acid and putrescine prevented the insolubilization of native collagen type III, by fibronectin and heparin.