The procoagulant material of lymphocytes has been characterized as tissue factor. Lymphocytes stimulated with phytohemagglutinin or the purified protein derivative of the tubercle bacillus developed procoagulant activity with incubation in tissue culture. While this material corrected the prolonged clotting time of factor VIII (AHF) deficient plasma, we have shown, utilizing a sensitive radioimmunoassay, that no AHF antigen was present in the cell cultures. Further, we have demonstrated this material to be tissue factor by coagulation techniques and immunological cross-reactivity. The published data regarding factor VIII synthesis is reviewed in light of these observations and comments are made regarding the role of the lymphocyte procoagulant.