The inactivation of thrombin by antithrombin III was studied in the presence of immobilized heparin. It was found that heparin accelerated the inactivation of thrombin by first forming a complex with thrombin which was then inactivated by antithrombin III contrary to the hypothesis of Rosenberg and Damus (J. Biol. Chem., 248, 6490, 1973) and the results of Salzman et al. Heparin was immobilized onto polyvinyl alcohol (PVA) by acetal coupling with glutaralde- hyde to give a very stable bond at pH 7.4. Using small columns of heparin-PVA beads eluted by 0.14M NaCl buffered at pH 7.4 both thrombin and antithrombin III bound to the immobilized heparin. If thrombin was loaded before an excess of antithrombin III, significant inactivation of thrombin was observed; however, loading antithrombin III before thrombin did not measurably inactivate thrombin. The results suggest that the covalently-bound heparin effectively participates in the inactivation of thrombin through the formation of surface bound heparin-thrombin, which then reacts with antithrombin III to yield a surface bound thrombin-antithrombin III complex. Heparin-PVA is potentially useful as a nonthrombogenic coating in the preparation of small diameter vascular prostheses and blood sampling catheters.