The objective of this study was to determine the role of the kallikrein- kinin system in healthy humans after intravenous administration of either Escherichia coli endotoxin or saline. We studied a total of 18 healthy nonsmoking volunteers, 23 to 38 years old, in an open-label study at the Critical Care Medicine Department, Clinical Center, National Institutes of Health (Bethesda, MD) in which some of the patients served as their own controls. After baseline data collection, the subjects received intravenously either E coli endotoxin (n = 15, 4 ng/kg of body weight) or saline (n = 8, controls). Signs, symptoms, systemic blood pressure, factor XII, plasma prekallikrein (PK), factor XI (FXI), antithrombin III (AT-III), high molecular weight kininogen (HK), and alpha 2-macroglobulin-kallikrein complexes were measured at baseline and 1, 2, 3, 5, and 24 hours after injection of either saline or endotoxin. After infusion of endotoxin, we found the functional plasma levels of FXI decreased at 2 hours (P < .05) and at 5 hours (P < .05). Functional PK was significantly depressed by 2 hours (P < .05), at 5 hours (P < .05), and at 24 hours (P < .01), whereas the PK antigen was only low at 5 hours (P < .05). These changes were accompanied by a significant increase in circulating alpha 2-macroglobulin-kallikrein complexes at 3 hours (P < .05) and 5 hours (P < .01). No significant changes occurred in the plasma levels of factor XII or HK. We concluded that clinical response to intravenous endotoxin in healthy human volunteers is associated with activation of the kallikrein-kinin systems. Further investigation is needed with specific inhibitors of the kallikrein-kinin system to define its primary or secondary role in the endotoxin-mediated reactions.