Thirty-four patients (31 female and three male patients) with a previous anaphylactoid shock to muscle relaxants were investigated. The seric antimyorelaxant IgE was detected by radioimmunoassay (RIA), and the results were compared to intradermal test (IDR) reactions to dilutions of the commercial drugs. The RIA was carried out with a Sepharose-myorelaxant solid phase and anti-IgE 125I-labeled IgG. The results corresponded to the percentage of labeled anti-IgE bound on the solid phase. The RIA with Sepharose-alcuronium and Sepharose-choline was estimated positive from determination with normal sera (n = 12) when bound IgE was greater than 1.0% and 1.5%, respectively. The RIA and IDR were positive in 43.5% and 75%, respectively, of the cases, with a concordance of 66%. One test at least was positive in 79.4% of the cases. No correlation was found between IgE seric levels and the RIA nor between the cutaneous sensitivity and the RIA. Cross-reactivity with Sepharose-choline and Sepharose-alcuronium was observed in 50%, and it was demonstrated by IDR in only 34.2%. The RIA demonstrated the specificity of IgE to quaternary ammonium compounds as myorelaxant drugs. The positive IDR revealed the bridging of mast cell-bound specific IgE, depending on structural conditions, such as the flexibility of the molecules or the variable specificity of the antibodies, restricted to quaternary ammonium ions or enlarged to a broader part of the incriminated molecules.