Intradermal skin tests (two dilutions) carried out systematically with 5 muscle-relaxant drugs were used to study the cross-reactivity between these drugs in 131 patients who had suffered from anaphylaxis. The skin tests were positive for at least 2 drugs in 65.6% of the cases. We observed a significant concordance (positive-positive or negative-negative skin tests) between pancuronium and vecuronium (p less than 0.01) and a significant correlation between the skin test diameters obtained with these 2 drugs (p less than 0.02 for wheals and p less than 0.01 for flares). Conversely, the lack of concordance between pancuronium and suxamethonium was significant (p less than 0.05). When comparing all the concordances obtained with each pair of drugs, it appeared that the concordance between pancuronium and vecuronium was significantly higher than all others. For other concordances, 2 main factors seemed to play a role: the N+ of the ammonium ions included in a ring as in pancuronium and the presence of a 'choline-like' side chain as in suxamethonium. But there was no significant correlation between skin test diameters. Altogether, these results suggest that in vivo there is no 'true' cross-reactivity between the drugs except frequently between pancuronium and vecuronium. In all other cases, the patients have probably specific IgE antibodies for each drug, all these antibodies being able to recognize the ammonium determinant; this common property may be insufficient to induce a clinical allergy.