Thrombin adheres tenaciously to glass and resists removal by repeated rinsing. This film of thrombin is sufficient to reduce the clotting time of a hemophilic blood from 30 min., as detd. in a clean test tube, to 10 min. This acceleration does not occur in the absence of platelets, which suggests that thrombin and platelets interact probably through a plasma factor, thromboplastinogen, to produce more thrombin.