Following the demonstration 25 years ago that substituted ammonium groups on neuromuscular blocking drugs (NMBDs) are the main allergenic structures recognized by IgE antibodies in the sera of some patients who experience anaphylaxis during anaesthesia, immunoassays for these drugs were quickly applied to supplement skin tests in the diagnostic assessment of suspected adverse reactions to anaesthetic agents. Many subjects who react to an NMBD do so on first exposure and this led to the speculation that the origin of allergic sensitization is an environmental agent(s) or another drug containing an ammonium ion. Direct antibody binding and hapten inhibition studies revealed that morphine, which contains a tertiary amino group, was strongly recognized by IgE in sera from anaphylactic patients and a morphine-solid phase immunoassay was found to be superior to NMBD-based assays for the detection of NMBD-reactive IgE antibodies. Extensive inhibition experiments indicate the likelihood of antibody combining site heterogeneity with recognition at the fine structural level of features additional, and adjacent to, ammonium ions. Further quantitative investigations are needed to identify these neighbouring groups on different NMBDs. Recent work has implicated the morphine analogue pholcodine as the sensitizing agent in Norway where, unlike Sweden, anaphylactic reactions to NMBDs are not uncommon and the medicament is available over-the-counter. This has led to the suggestion that allergenic sensitization to the ammonium group of pholcodine may account for the different incidences of anaphylaxis during anaesthesia in the two countries. This work is subjected to critical review and some alternative speculations on the nature and origin of the sensitizing agent(s) are presented.