Blood represents a complex system containing plasminogenn activators. At present, three different types of plasminogen activator activity are distinguished. It has been assumed that urokinase is not one of the blood activators, but recently an improbably high concentration of urokinase antigenic material has been claimed to be present in plasma.The present study was performed to establish if urokinase activity participates in blood fibrinolysis. This was investigated by measuring quenching by

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... tibodies. The dextran sulphate euglobulin fraction (DEF) of plasma was prepared and its fibrinolytic activity was measured in a clot lysis time method with and without addition of various anti-urokinase immunoglobulin fractions (AUK's).Considerable quenching of the DEF activity by the AUK's was found. The quenchings reached maximum values at high antibody concentrations. The quenching curves of the fibrinolytic activity of both urokinase and DEF could be superimposed for each AUK. This quenchable activity was also very sensitive to inhibition by a specific urokinase inhibitor from human placenta. Addition of kaolin to DEF, to include factor XII-dependent fibrinolysis, showed an increase of the fibrinolytic activity which was not quenched by AUK. Double diffusion analyses did not result in the demonstration of urokinase antigen in plasma. This was in agreement with the amount of activity quenched (comparable with about 1 ng urokinase/ml plasma) and the detection limit for a visible precipitation line.These observations indicate that plasma contains urokinase-related activity. This activity is assumed to belong to the factor XII-independent proactivator system.