A case is reported of anaphylactic shock due to vecuronium occurring in a patient who had already had such a shock, due then to pancuronium, during a previous general anaesthesia. The need for a full immuno-allergological investigation, the occasional efficiency of the anti-histamine premedication, and crossed allergies between muscle relaxants are stressed. It is noted that an anaphylactic shock can be seen on first using a new molecule, as the patient can have been sensitized to it by other muscle relaxants. This case was the first to be described of an anaphylactic shock due to vecuronium bromide.