Blood was collected with special care and centrifuged in silicone treated glass tubes at 22-23,000 R.P.M. for various periods of time. Even after prolonged centrifugation, incoagulable plasma could not always be obtained. When clots formed, only a small portion of the fibrinogen was represented as fibrin. Minute amounts of thrombin cause this fibrin formation. No evidence was found to support the view that plasma may contain a soluble factor which can initiate clotting independent of platelet action.