The accelerating effect of foreign surfaces on coagulation of whole blood has been recognized for many years. In 1863, Lister (1) was aware of the great influence exerted upon the blood by exposure for a very short time to a foreign solid . . . ," and concluded, "The real cause of the coagulation of the blood, when shed from the body, is the influence exerted upon it by ordinary matter. . . ." Since the original observations of Lister, many subsequent investigators have studied this phenomenon (2-15).In the experiments to be described, the rate of prothrombin utilization was employed as a measure of the effect of foreign surface on the blood coagulation mechanism. Blood and plasma from normal human subjects and from patients with hemophilia and thrombocytopenia were exposed to differing glass surface areas, and prothrombin utilization was measured. METHODSMethods of obtaining and handling blood: Approximately 60 ml. of blood were drawn from the antecubital vein of human subjects using sterile, non-silicone treated 18 gauge needles and silicone 4 treated syringes lubricated with silicone oil.5 In order to expose this blood to various areas of foreign surface, 2.0 ml. portions of the blood were measured in duplicate directly from the syringe into 13 x 100 mm. test tubes which had been prepared in different ways. Some tubes were silicone-treated, others were plain glass (i.e., chemically clean, dry, "Pyrex"