α₂-Antiplasmin is the principal plasma inhibitor of plasmin, with which it rapidly and irreversibly forms a complex. The simple technique of crossed immunoelectrophoresis against antiserum to α₂-antiplasmin has been applied to detect plasmin-α₂-antiplasmin complex in plasma. The presence of this complex is demonstrated in patients in whom overactive fibrinolysis was the only or major contributor to severe haemorrhagic disorders arising spontaneously.

When streptokinase-activated, urokinase-activated, or spontaneously activated human plasmin is mixed with increasing amounts of human antiplasmin, caseinolytic effect decreases much more rapidly than fibrinolytic effect. At concentrations of antiplasmin at which caseinolytic effect completely disappears, considerable fibrinolytic activity persists.