With the now widespread and extensive use of muscle relaxant drugs in general anesthesia, a steadily increasing number of cases of anaphylactic-like reactions is being reported (1-3). Muscle relaxants seem to be responsible for half of the adverse reactions occurring during general anesthesia (4). The muscle relaxants predominantly implicated are d-tubocurarine, alcuronium, gaUamiue, suxamethonium, pancuronium, vecuronium, and more recently atracurium. Alcuronium is mostly involved in Australia, whereas anaphylactic shocks are rather provoked by suxamethonium in France. Clinical symptoms vary from mild cutaneous erythema to major life-threatening cardiovascular collapse and/or bronchospasm. It is clear that the well-known histAmine-releasing properties of neuromuscular blocking agents cannot account for all adverse reactions observed. Fifty percent of subjects who experienced anaphylaxis after the administration of muscle relaxants may be sensitive to one or more of these agents. In these cases, classical evidence of Type I hypersensitivity (i.e., anaphylaxis) is found, and these patients usually demonstrate exquisite intradermal sensitivity to the injection of diluato relaxant solutions. The presence ofseric-specific IgE against muscle relaxants has been demonstrated, thus providing an opportunity to look for serological crossreactivity by using the compounds in radiolmmunoassay inhibition experiments. The antigenic group of muscle relaxants lies in the quaternary or tertiary ammonium group. These groups also confer neuromuscular-blocking properties *Author to whom all correspondence and reprint requests should be addressed. C/inica/ Reviews in Allergy 281