SurnrnuryWe studied the release in vivo of platelet specific proteins platelet factor 4 (PF4) and /l-thromboglobulin (PTG), and confirmed that only PF4 is released, after heparin intravenous administration. A good correlation was found between platelet count and PF4 and this seems at variance with the idea that the source of released PF4 is vascular endothelium rather than platelets; however, such a possibility cannot be excluded. Although platelet function was blocked by aspirin injection heparin was still able to induce the release of PF4 and exerted a potentiating effect on platelet aggregation induced by ADP and collagen.