A 77-year-old man was admitted for mitral valve replacement, 46 days after a failed conservative mitral surgery where he received high-dose aprotinin. Twenty minutes after induction of anaesthesia, 250 UPh E of aprotinin were infused intravenously; before the end of this infusion, bronchospasm, systemic hypotension and generalized rash were noted. Immediate treatment included intravenous adrenaline and methylprednisolone; cardiovascular stability was restored after 10 minutes. Immediate histamine liberation was confirmed by the analysis of the time course of the clinical events, a previous contact and positive skin tests. Aprotinin has the antigenic molecular structure of natural proteins. Since 1987, it is used in cardiac surgery to reduce postoperative blood loss: to prevent serious allergic reactions to aprotinin, it is necessary, in patients known to have had previous aprotinin therapy, to perform skin testing with diluted aprotinin before infusion.