In order to clarify whether adverse reactions to radiographic contrast media may be allergic in nature, it is essential to show that contrast media can act as haptens and induce antibody formation in vivo. Experiments in Hooded Lister rats have been carried out using techniques specifically designed to produce favourable conditions for the production of antibodies. No evidence of any antibody formation was found in any of the experiments. These experiments suggest that radiographic contrast media in their native form do not act as haptens, contrary to previously published data, and it therefore seems unlikely that the majority of adverse reactions to radiographic contrast media are allergic in nature.