Potentially life-threatening immediate hypersensitivity reactions are extremely rare among patients treated with thrombolytic agents for suspected acute myocardial infarction. A patient who developed a severe reaction during an infusion of recombinant tissue-type plasminogen activator is described. Potential causal mechanisms for the reaction could be related either to nonmedicinal additives or complement activation. Implications for treatment in the setting of acute myocardial infarction are discussed.