Isolated human monocytes generate tissue factor when stimulated with endo toxin. Tissue factor generation provides a marker for activation of the monocyte and of the clotting system. Determination of the recalcification time of blood after incubation with endotoxin detects minute changes in coagulability. This clotting assay was utilized to assess the presence of a hypercoagulable state in patients with peripheral arterial occlusive disease when compared with healthy volunteers. Citrated blood was incubated with endotoxin for two hours, CaCl2 was added, and the recalcification time determined. Hypercoagulability was indicated by shortened recalcification time. The recalcification time ± standard deviation for saline (control) and endotoxin-activated samples from 19 healthy volunteers was 6.55 ±0.8 and 5.69±0.7 minutes, respectively, whereas it was 4.93±1.2 and 4.55 ±0.9 minutes for 31 patients with peripheral arterial occlu sive disease (p < .001 for each). This hypercoagulable state can accentuate the arterial occlusive process in patients with peripheral vascular disease and may prove to be of diagnostic, therapeutic, and prognostic significance.