Neutralization of streptokinase by an unsuspected high titer of antistreptokinase antibody prevented activation of the fibrinolytic system and induction of a lytic state in a 62 year old man with an acute inferior myocardial infarction. There was no decrease in serum fibrinogen, minimal decrease in serum plasminogen and only a small increase in serum fibrin degradation products after intravenous administration of 1.5 million units of streptokinase. A high titer of antistreptokinase antibody, sufficient to neutralize 1.5 million units of streptokinase, was demonstrated by semiquantitative counterelectrophoresis . There was no clinical evidence of coronary artery reperfusion, and coronary angiography confirmed complete occlusion of the left circumflex artery.