The venom of the Mojave rattlesnake, Crotalus scutulatus scutulatus , was fractionated as previously described ( Pattabhiraman and Russell , 1975). Fraction K′ ( C.s.s. Protein K′), having a molecular weight of ca. 20,000 and an ld 50 of 0·03 mg/kg, was studied on the extensor digitorum longus muscle preparation of the frog. The fraction caused a significantly delayed depression of the indirectly elicited end plate potentials, with quantal content and miniature end plate potential frequency falling rapidly between 25 and 50 min after the application of the toxin. The findings indicate a presynaptic site for the action of protein K′.