A LTHOUGH the question of the susceptibility of snakes to their own venom, or to venom of other species, has been of interest for some time, relatively little experimental work has been undertaken in this field. Most reports in the literature cite examples in which one snake was seen to bite another. In some of these the bitten animal died, in others no effects were observed. It was not possible to estimate the amount of venom injected with such a bite. Gloyd (1933), Wooster (1933), and Conant (1934), have published such reports. On several occasions in the laboratory, the authors have seen poisonous snakes bite non-venomous species, with similarly varying results. The results of injection of measured amounts of the venoms of North American crotalids into 31 snakes, representing 20 species, from several regions of the United States, are here reported. Weir Mitchell (1861), and Nichol, Douglas and Peck (1933) have obtained evidence that several North American crotalids are not immune to their own venoms if sufficiently large quantities are injected. Noguchi (1904), found that the green snake (Opheodrys vernalis) showed almost no symptoms when given an injection of 5 mg. of Crotalus venom. MATERIALS AND METHODS