A study was carried out on mechanisms, independent of activated Factor XI, capable of activating Factor IX. The reaction product of tissue factor and Factor VII functioned as a potent Factor IX activator in the assay system used. Activated Factor IX itself activated Factor X; thrombin failed to activate Factor IX. Sodium dodecyl sulfate/polyacrylamide gel electrophoresis confirmed that the reaction product of tissue factor and Factor VII activated Factor IX, with replacement of the band corresponding to native factor IX [molecular weight (Mr) 55,000] by bands corresponding to the heavy chain (Mr 27,000) and light chain (Mr 17,000) of activated Factor IX. When either Factor VII or calcium ions were left out of incubation mixtures, the band of native Factor IX persisted unchanged. Contact of blood with tissue factor represents a second mechanism, bypassing activated Factor XI, for the activation of Factor IX during hemostasis. It may help to explain the discrepancy between the mild bleeding of hereditary Factor XI deficiency and the severe bleeding of hereditary Factor IX deficiency.