Molecules that induce tissue factor expression by responsive cells such as endothelial cells and monocytes may be important in the regulation of hemostasis and, perhaps, in mediating certain hemostatic disorders. A constituent of normal human plasma capable of inducing tissue factor activity in human endothelial cells and monocytes has been isolated and identified as a derivative of, or modification associated with albumin. Procoagulant albumin caused a concentration-dependent induction of tissue factor expression by human endothelial cells, but bovine endothelial cells were unresponsive. The dose-response curve developed a plateau phase, indicating that the capacity of endothelial cells to respond to the stimulus was finite. The maximum response induced by the procoagulant albumin was similar to that observed for maximally effective concentrations of endotoxin, interleukin-1, and tumor necrosis factor. Time-course studies showed that procoagulant albumin produced peak activity in 4 to 6 hours. Identification of a procoagulant form of albumin in normal human plasma suggests a potential role for this constituent in regulation of hemostasis.