A polemic in reply to C. Ts'ao and T. S. Gallozo (ibid. 1981, 75, 372-377).The data presented indicate that the American Dade Protopath Plasminogen assay measures plasminogen as the streptokinase-plasmin complex but this does not affect the validity of the assay for plasminogen.The substrate is not insensitive to plasmin but rather is hydrolyzed more easily by streptokinase-plasmin or streptokinase-plasminogen activators than by free plasmin.The kit is not suitable for plasmin determination in plasma and is marketed solely as a plasminogen assay.