In fibrinolysis a major route of fibrin degradation involves proteolytic fragmentation by the serine protease plasmin. Tissue-type plasminogen activator (t-PA) is a serine protease capable of activating plasminogen to plasmin by limited proteolysis. The specific and strong stimulation of the plasminogen activation by t-PA when it takes place in the presence of fibrin provides an argument for specific involvement of t-PA in fibrinolysis. The predominant source of t-PA for hemostasis is the endothelial cell. Endothelial cells can provide the t-PA by continuous synthesis and possibly by acutely evoked, stimulated release. Circulating t-PA is continuously available to existing vascular fibrin deposits. In vitro, preformed plasma clots can rapidly take up t-PA from the surrounding medium, an uptake proportional to the t-PA concentration. At very high t-PA concentrations, significant plasminogen activation can be expected and may explain systemic plasminogen activation in patients.