An antiserum was raised in rabbits against human plasmin-antiplasmin complex and rendered specific for neoantigens of this complex by absorption with purified plasminogen and plasma. Polystyrene particles were coated with the specific antibodies and used in an agglutination test for the determination of plasmin-antiplasmin complex in the plasma from various patients. Purified plasmin-antiplasma complex at a concentration of 0.1-0.2 mg/l was found to cause a clear agglutination of the particles. Activation of fresh human plasma with urokinase caused progressive generation of agglutinating activity up to a plasma dilution of 1/480. Intravenous infusion of streptokinase into patients resulted in an increase of the plasmin-antiplasmin titre of at least 1/240. Sera from patients with rheumatoid factor also agglutinated the particles but this activity could be removed by absorbing rheumatoid factor on insolubilized human IgG. Out of 101 male and twenty-three female control subjects, only three men had a plasmin-antiplasmin titre above 1/16. Of 230 hospitalized patients, plasmin-antiplasmin titres of 1/40 or more were detected in twenty-five patients. Most of these patients had diseases which are frequently associated with in vivo coagulation or fibrinolysis, but among them there was only one who showed diffuse intravascular coagulation detectable by classical methods. In the absence of an increased plasmin-antiplasmin titre none of the haemostasis analyses were indicative of in vivo coagulation or fibrinolysis. Seven out of eight patients with diffuse intravascular coagulation of various origin had plasmin-antiplasmin titres of 1/80 or 1/160. Thus, the present latex agglutination test, owing to its simplicity and sensitivity, appears to be a practical routine screening test for detecting fibrinolytic activation in plasma.