Human plasma fibronectin (FN) was reduced and carboxamidemethylated, and its binding ability to several matrices was analyzed in vitro. The binding of S-carboxamidemethyl (Cam)-FN to heparin-Sepharose was not influenced by either 4 M urea, 0.5 M NaCl or 0.5% heparin, but was disrupted by the coexistence of urea and NaCl or heparin. S-Cam-FN, compared with intact FN, obviously had a more potent ability to bind heparin, while it had little or no binding ability to gelatin, fibrin and thrombin-stimulated platelets. A conformational change of S-Cam-FN by heparin-binding has been proposed as a possible mechanism from the result of circular dichroic spectrum measurement.