Albumin and fibrinogen show no binding affinity to varied molecular weights of heparin at physiological pH. Human plasma fibronectin was shown to bind heparins in both the solution and adsorbed states. Fibronectin was shown to have six active binding sites for heparins which may be sterically blocked in some adsorbed states. 125I-Fibronectin monolayer concentrations were shown to be significantly different on polyvinyl chloride surfaces when compared to hydrophilic/hydrophobic silica, Biomer, Silastic, and polystyrene surfaces. Preadsorbing fibronectin to various substrates and then allowing heparins to interact with the protein monolayer made it possible to bind up to 0.2 μg/cm2 heparin in a plasma environment. This fibronectin-heparin complex was at least 85% stable in plasma and buffer solutions for up to 8 h time. The complex was observed to prolong blood clotting times two to three times over that of controls as assayed by Activated Partial Thromboplastin Times. All of the bound heparin was observed to be active by its ability to bind Factor Xa in plasma. Monolayers of blood proteins adsorbed from human serum were not observed to be active in binding heparins. The fibronectin-heparin conjugate showed low activation of blood components compared to protein monolayers preadsorbed from human sera as assayed by Activated Partial Thromboplastin Time.