Three proteinaceous toxins (named echotoxins 1, 2, and 3) with both lethal and hemolytic activities were purified from the salivary gland of the marine gastropod (Monoplex echo) by gel filtration on Sephadex G-75 and cation-exchange FPLC on Mono S, although marked reduction in toxicity was observed in the latter chromatography. When subjected to reverse-phase HPLC on TSKgel Phenyl-5PW RP, echotoxin 2 afforded a single peak, while both echotoxins 1 and 3 were further separated into two components, respectively. All echotoxins are monomeric simple proteins with a molecular mass of about 25kDa and their N-termini are blocked. Amino acid compositions of echotoxins 1a, 1b, and 2 are closely similar to one another, being characterized by the abundance of Gly and Ala and the absence of half-Cys, Met, and Trp. Of the sugars, lipids and glycolipids tested, gangliosides potently inhibited the hemolytic activity of echotoxins. Furthermore, ganglioside G(T1b) inhibited both in vivo hemolysis and lethality in mice displayed by echotoxins. It is thus concluded that echotoxins injected into mice induce rapid hemolysis by binding to receptors (possibly gangliosides) gangliosides in the erythrocyte membrane, eventually leading to death of mice.