The fibrinolytic system in mammalian blood comprises an inactive proenzyme, plasminogen, which is activated to the proteolytic enzyme plasmin by two immunologically distinct physiologic plasminogen activators (PA), tissue-type plasminogen activator (t-PA) and urokinase-type plasminogen activator (u-PA). Inhibition may occur either at the level of the PA, by specific PA inhibitors (PAI), or at the level of plasmin, mainly by ~_-antiplasmin. t-PA is primarily responsible for removal of fibrin from the vascular tree; it ~as a specific affinity for fibrin and produces clot-restricted plasminogen activation. Impairment of fibrinolysis, associated with thrombosis, may be due to a defective synthesis and/or release of t-PA from the vessel wall, to a deficiency or functional abnormality in the molecular interactions regulating plasminogen activation or to increased levels of inhibitors of t-PA or of plasmin. On the other hand, excessive fibrinolysis due to increased levels of t-PA or to deficiency of ~2-antiplasmin or PAI, may result in a bleeding tendency, u-PA lacks affinity for fibrin and requires conversion from a single chain to a two chain molecule to be fully active, u-PA binds to a specific cell-surface receptor and is believed to be primarily involved in cell-mediated proteolytic events such as wound healing, macrophage invasion, ovulation, angiogenesis and tumor invasion. In m~n, PAI-I deficiency results in delayed rebleeding; genetic deficiencies of t-PA or u-PA have not yet been reported. Recently, the effect of PAI-I gene inactivation on hemostasis, thrombosis and thrombolysis was studied in homozygous PAI-l-deficient mice, generated by homologous recombination in embryonic stem cells. Disruption of the PAI-I gene appeared to induce a mild hyperfibrinolytic state and a greater resistance to venous thrombosis, but not to impair hemostasis. This may suggest some phenotypic dissimilarity between mouse ~nd man. Furthermore, the role of t-PAand u-PA-mediated plasminogen activation in development, reproduction, thrombolysis, thrombosis and macrophage function has been studied in mice with single and combined inactivation of the t-PA and/or u-PA genes. Mice with single deficiencies of t-PA and u-PA developed normally, were fertile and bad a normal life span. t-PA-deficient mice bad a reduced thrombolytic potential, whereas u-PA-deficient mice occasionally developed spontaneous fibrin deposits in tissues and displayed deficient plasmin-mediated macrophage function. Mice with combined deficiency of t-PA and u-PA survived embryonic development, but showed extensive fibrin deposition and suffered retarded postnatal growth, reduced fertility and shortened life span. These findings indicate that t-PA and u-PA have divergent roles in fibrinolysis and macrophage function and that they may cooperate in complex biological processes.