Collagen-fibronectin complexes, formed by binding of fibronectin to gelatin or collagen insolubilized on Sepharose, were found to bind 20-40% of radioactivity in [35S]heparin. Fibronectin attached directly to Sepharose also bound [35S]heparin, while gelatin-Sepharose without fibronectin did not. Unlabeled heparin and highly sulfated heparan sulfate efficiently inhibited the binding of [35S]heparin, hyaluronic acid and dermatan sulfate were slightly inhibitory, while chondroitin sulfates and heparan sulfate with a low sulfate content did not inhibit. The interaction of heparin with fibronectin bound to gelatin resulted in complexes which required higher concentrations of urea to dissociate than complexes of fibronectin and gelatin alone. Heparin as well as highly sulfate heparan sulfate and hyaluronic acid brought about agglutination of plastic beads coated with gelatin when fibronectin was present. Neither fibronectin nor glycosaminoglycans alone agglutinated the beads. It is proposed that the multiple interactions of fibronectin, collagen and glycosaminoglycans revealed in these assays could play a role in the deposition of these substances as an insoluble extracellular matrix. Alterations of the quality or quantity of any one of these components could have important effects on cell surface interactions, including the lack of cell surface fibronectin in malignant cells.