A hematologist's view of contrast media, clotting in angiography syringes and thrombosis during coronary angiography. While ionic contrast media (CM) are stronger anticoagulants and antiplatelet agents, both nonionic and ionic CM retard clotting, fibrinopeptide A generation and platelet aggregation (at least by Born-O'Brien aggregometry). Thus, nonionic CM do not cause clots and thrombi. Rather, the driving force for clot or thrombus formation, when it occurs, is blood contact with and activation by the foreign surface of a syringe or catheter itself. A marked enhancement of clotting by glass syringes in comparison to plastic ones supports this view. Blood in any syringe or catheter, therefore, will clot more slowly in the presence of nonionic or ionic CM, the inhibitory effects of the latter being more profound. With respect to models of thrombosis at sites of vascular injury or stenosis, the antithrombotic effects of CM may either be transient owing to the dynamic nature of blood flow (local endothelial cell denudation model), or as in the case of ionic CM, actually to enhance local platelet aggregation (stenosis model). In these situations, preservation of the antithrombotic functions of endothelium with nonionic CM may be quite critical. 