A homologous series of eight quaternary ammonium salts (quats) were used as complex cations in a survey of contact hypersensitivity in guinea pigs. Two of the quats tested were found to be strong allergens which was due to stable association with membrane lipids at the surface of epidermal cells. This surface complexation reaction was studied in detail by using a spin-labelled quat of intermediate allergenicity. Electron spin resonance was used to show that stable "ion pairs" are formed between membrane receptor sites and the two strong allergens. Information was obtained on the specificity and kinetics of immunogenic complex formation as well as on the position and orientation of these haptens on epidermal receptor sites in vivo.