Background Platelets contain several factors that inhibit heparin. This study was designed to assess the heparin-neutralizing activity present in acute, platelet-rich arterial thrombi formed at sites of arterial injury in animals.Methods and Results Platelet-rich thrombi (n=3) were induced in pig coronary arteries by balloon catheter-mediated arterial injury. Soluble extracts were prepared from each thrombus and assayed for the capacity to inhibit heparin in an in vitro clotting assay (activated partial thromboplastin time). Mean heparin-neutralizing activity was 28 U of heparin neutralized per milliliter of thrombus, indicating that 1 vol of coronary thrombus completely inhibited the heparin present in 140 vols of therapeutically anticoagulated (0.2 U heparin/mL) plasma. In contrast, thrombus extracts had no effect on the anticoagulant activity of hirudin, a direct-acting thrombin inhibitor. The heparin-neutralizing activity present in coronary thrombi bound to heparin-agarose and was eluted from it by 1.4 mol/L NaCl, suggesting that platelet factor 4 mediated the antiheparin effect of thrombi. Consistent with this hypothesis, a murine monoclonal antibody to rabbit platelet factor 4 nearly completely inhibited the heparin-neutralizing activity present in rabbit thrombi (n=3) generated by carotid artery injury.Conclusions Extracts prepared from platelet-rich arterial thrombi significantly inhibit the in vitro anticoagulant potency of heparin but not of hirudin. This antiheparin effect appears to be mediated by platelet factor 4. These results are consistent with the hypothesis that localized inhibition of heparin at sites of platelet activation may reduce its antithrombotic efficacy. In addition, they suggest an additional mechanism for the apparent superiority of hirudin over heparin as a thrombin inhibitor at sites of arterial injury. (Circulation. 1994;89:1523-1529.)