Endotoxemia was evoked by bolus injection of Escherichia coli endotoxin (2 ng/kg body weight) in six healthy subjects to investigate the early kinetics of cytokine release in relation to the development of clinical and hematologic abnormalities frequently seen in gram-negative septicemia. The plasma concentration of tumor necrosis factor (TNF) increased markedly after 30 to 45 minutes, and reached a maximal level after 60 to 90 minutes. In each volunteer, the initial increase of plasma interleukin 6 (IL-6) concentrations occurred 15 minutes after the initial TNF increase, and maximal IL-6 concentrations were reached at 120 to 150 minutes. A transient increase in body temperature and pulse rate occurred simultaneously with the initial TNF and IL-6 increases, whereas a significant decrease in blood pressure occurred after 120 minutes. These changes were proportional to the changes in TNF and IL-6 concentrations. Coagulation activation, as assessed by a rise of prothrombin fragments and thrombin-antithrombin III complexes, was noted after 120 minutes, in the absence of activation of the contact system. A two- to sixfold increase in the concentrations of tissue plasminogen activator (t-PA) and von Willebrand factor antigen indicated endothelial cell activation. This increase started at 120 and 90 minutes, respectively. The release of t-PA coincided with activation of the fibrinolytic pathway, as measured by plasmin-alpha 2-antiplasmin complexes. The fibrinolytic activity of t-PA was subsequently offset by release of plasminogen activator inhibitor, observed 150 minutes after the endotoxin injection, and reaching a peak at 240 minutes. No complement activation was detected. These results show that in humans endotoxin induces an early, rapidly counteracted fibrinolytic response, and a more long-lasting activation of thrombin by a mechanism other than contact system activation. In addition, our data suggest that endotoxin-induced leukopenia and endothelial cell activation are mediated by TNF.