Antiplasmin activity has been determined by means of the plasmin specific trlpeptlde substrate H-D-Vai-L-Leu-L-Lys-p-nltroanlllde (S-2251)It Is concluded that the method mainly measures the antiplasmin described by Collen and coworkers ( Thromb. Res. 7, 245, 1975), although α2-macroglobulln may interfere to a small extent. Different categories of patients with tendency to thrombosis and Increased fibrinolysis have been studied During normal mlddlesized operations antiplasmin decreased slightly, but increased postoperatively. Antiplasmin likewise decreased during deliveries, and Increased In th PuerperiumPatients with acute deep vein thrombosis had normal antiplasmin activity at the time for diagnosis, but antiplasmin Increased later during the course of eventsStreptokinase treated patients showed a pronounced and rapid drop In antiplasmin activity, which was normalized as soon as the streptokinase administration was InterruptedPatients with liver cirrhosis had lower antiplasmin activity than normals , and so had patients with disseminated Intravascular coagulation.