Incubation of plasma from rats pretreated with tranexamic acid (40 mg/l 00 g) with acetone (23 % v/v) yielded enzyme preparations in which all the plasminogen present was recovered as plasmin and a plasmin‐like substance without affinity for lysine‐Sepharose. This substance, designated “plasmin”, was separated from plasmin and kallikrein in a three‐step procedure using columns of lysine‐Sepharose, DEAE‐Sephadex A‐50, and arginine‐Sepharose. The ratios of fibrinolytic, caseinolytic, LEe esterase, BAEe esterase and kininogenase activities of “plasmin” corresponded well with those of rat plasmin and human plasmin. Both rat plasmin and “plasmin” destroyed the capacity of high molecular weight kininogen (HMWK) to function as a cofactor in the activation of factor XII in rat plasma, without causing a corresponding release of the kinin part of the molecule. Rat plasma kallikrein induced full release of kinin from HMWK, but the functional capacity was retained. It is suggested that the reduced extent of activation of factor XII observed in plasma from rats injected intravenously with dextran, or rat plasma that has been passed through a column with lysine‐Sepharose, is due to the loss of functional HMWK caused by plasmin activated in vivo or on the column.