The vascular lesions with microthrombi were studied in 12 patients with thrombotic thrombocytopenic purpura (TTP), diagnosed by the characteristic clinical and laboratory findings and confirmed histologically in each case. While defibrination was not observed, and with only minimal changes in the circulating levels of fibrinogen, fibrin degradation products and plasminogen activator, the microthrombotic lesion was invariably present. Immunofluorescent and histochemicaL studies indicated that both platelet and fibrin were present In the mitrothrombi with the platelet components dominant In many cases. Using the fibrin slide method, plasminogen activator was demonstrated in the uninvolved blood vessels but totally absent in the vessels occluded by microthrombi. In contrast, fibrinolysis is always present in the vessels afflicted with other types of thrombosis, such as the microthrombi in disseminated intravascular coagulation. Since circulating fibrinolytic activity was normal in TTP, the absence of vascular fibrinolysis is a local defect due to either inhibition by the platelet deposits or by local vascular damage. The inability of thrombolysis may explain the absence of systemic defibrination and the severity of the disease.