A comparative analysis of the primary and secondary structures of the neurotoxins mast cell degranulating peptide apamin and charybdotoxin, and of the hormone endothelin revealed a strikingly homologous structural element consisting of an ?-helical stretch spanning the sequence portion Cys X-X-X Cys and stabilized by disulfide bridging to a second consensus sequence Cys X Cys, itself folded in a ?-type structure. This structural motif generates a certain degree of amphiphilicity for the folded molecules which may be correlated with the bioactivities of the neurotoxins at the membrane level. The presence of the identical structural motif in endothelin raises the question of whether it may represent, even for this hormone, the clue for its biological activity.