In this paper the immobilization of small unilamellar DMPC/GM1 lipid vesicles containing a water-soluble bodipy dye is described. The binding of the complete alpha beta toxin expressed by Vibrio cholerae to the attached vesicles was measured using Surface Plasmon Resonance (SPR) and a value of the dissociation constant K-D obtained. Further measurements showed that the interaction of both the alpha beta-toxin and the beta-subunit alone resulted in the permeation of the lipid membrane, with release of a fluorophore contained within the vesicle being measured by combined SPR and Surface Plasmon enhanced Fluorescence Spectroscopy (SPFS). The leakage of dye through the membrane, measured by following the change in fluorescence, was fitted to a simple diffusion model. Finally, SPFS measurements of the effect of europium(III) chloride (EUCl3) showed that cholera toxin binding and subsequent membrane permeation could be blocked by 1 mu mol dm(-3) europium chloride. In view of the low oral toxicity of europium chloride, we speculate on the potential pharmaceutical applications of this molecule in the treatment of cholera infection.