SummaryBoth the thrombin-antithrombin III and plasmin-antiplasmin complexes contain antigenic structures which are not present in the parent molecules, to which precipitating and non-crossreacting antibodies can be prepared.Using a simple latex agglutination test, in which polystyrene particles were coated with specific immunoglobulins, these complexes were measured in purified systems and in the blood. The purified precursor molecules were at least 100 times less reactive than the complexes. Normal human plasma had agglutination titers of less than 1/8 in both tests and this value increased tot approximately 1/500 following activation of the coagulation or fibrinolytic system. In the plasma of patients during thrombolytic therapy, the plasmin-antiplasmin titer increased to values of approximately 1/250 and disappeared with a half-life of several hours.In patients with overt signs of diffuse intravascular coagulation, markedly increased titers for both complexes were consistently observed. These results indicate that activation of the coagulation or fibrinolytic system in vivo is associated with the appearance of circulating thrombin-antithrombin III or plasmin-antiplasmin complexes, which can be directly assayed in plasma on the basis of their neoantigenic expression.