A cardiotoxin from "milked venom" of the sea wasp (Chironex fleckeri) was purified by immunochromatography on an immobilized mouse monoclonal anti-Portuguese man-o'war (Physalia physalis) venom antibody column. The 20,000 molecular weight toxin caused bradycardia followed by cell lysis when applied to cultured chick embryonic cardiocytes at concentrations higher than 1.7 micrograms protein per ml and was lethal to mice at 0.04 micrograms protein per g. The toxin affected ion permeability in lipid bilayer membranes by forming monovalent cation channels.