The coagulation enzyme thrombin, a serine protease like all other coagulation factors, plays a central role in the hemostatic processes engaged after injurious events. It induces, with particular efficacy, the aggregation of blood platelets (primary hemostasis) and accounts, via splitting of fibrinogen to fibrin, for the event actually responsible for the coagulation of blood (secondary hemostasis). As is well-known, thrombin itself is generated by a cascade of activation events involving various coagulation factors (F). In this respect the "tissue factor" (TF, formerly known as thromboplastin), in combination with F VIIa, attains decisive significance, not only in the extrinsic pathway of coagulation (activation of F X-->Xa), but also in the intrinsic pathway (activation of F IX-->IXa). Under physiological circumstances, platelet aggregation and coagulation are restricted to the area of the vascular lesion, since the surrounding intact endothelium inhibits an intraluminal spreading of both processes. These "antithrombotic" features of the endothelium encompass antiaggregatory mechanisms (formation and release of prostacyclin [PGI2], adenosine, EDRF [NO], degradation of ADP and other nucleotides mediated by ecto-nucleotidases) as well as anti-coagulatory properties (formation and release of "tissue factor pathway inhibitor" [TFPI], which blocks the coagulation cascade by joining F Xa, TF and F VIIa into an inactive complex, thrombomodulin--thrombin induced activation of protein C, which, together with protein S, inactivates F Va and F VIIIa, thereby attenuating further generation of thrombin, and the heparan sulfate-enhanced activation of antithrombin III and heparin-cofactor II).(ABSTRACT TRUNCATED AT 250 WORDS)