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Two polypeptide toxins from scorpion ( Leiurus quinquestriatus) venom which block distinct classes of calcium-activated potassium channels have been identified and partially purified. One toxin, at 50–100 ngml, blocks apamin-sensitive potassium fluxes in hepatocytes and inhibits [ 125I]monoiodoapamin binding. The other, more basic, toxin blocks apamin-insensitive potassium fluxes in erythrocytes at 200 ngmland, to our knowledge, is the first toxin shown to block the erythrocyte calcium-activated potassium channel with high affinity. The possible co-identity of this latter toxin with charybdotoxin is discussed.