It has been known for many years that the effects of heparin are antagonized by platelets. This effect has generally been attributed to the release of platelet factor 4 (PF4). Heparin has recently been shown to disrupt the interactions of vitamin K-dependent clotting factors with phospholipid. This is due in part to the binding of heparin to phospholipid. The present investigations therefore explored the possible effects of phospholipid (cephalin) and degranulated platelets on the interaction of thrombin or factor Xa with antithrombin-III in the presence of heparin. These interactions were determined by the measurement of the second order rate constants for the inactivation of purified factor Xa or thrombin by purified antithrombin-III. The rates of inactivation of thrombin or factor Xa by antithrombin-III increased with increasing concentrations of heparin. Cephalin reduced the magnitude of the heparin effect on factor Xa but had no additional effect on thrombin inactivation. Relative to cephalin, gel filtered platelets reduced further the ability of heparin to enhance the inactivation of factor Xa by antithrombin-III. When the platelets were depleted of PF4 by degranulation with thrombin (2u/ml), the effects of platelets on the protease- antithrombin-III-heparin reactions approached those obtained with cephalin. However, no detectable effect on the inactivation of factor Xa or thrombin by antithrombin- III was demonstrable by gel filtered platelets, degranulated platelets, or cephalin in the absence of heparin. These results indicate that in addition to the contribution made by the release of PF4 by platelets, the calcium dependent binding of factor Xa to cephalin and platelet phospholipid is capable of protecting factor Xa from inactivation by antithrombin-III and heparin.