Blood thromboplastin was prepared from thromboplastin generation test reagents. It was found that activity could be sedimented by centrifugation at 20,000 rpm, and that the sedimented reagent clotted normal, factor V-deficient, and Stuart factor-deficient plasmas equally well. Human brain cephalin used as the source of phosphatide did not adsorb antihemophilic factor or plasma thromboplastin component non-specifically, and no activity could be sedimented in the absence of Ca ion. When the thromboplastic sediment was inactivated by incubation with serum, activity could not be restored by resedimentation and repeated washing.