The venoms of three vipers, Vipera palaestinae, Echis colorata and Pseudocerastes fieldi, and the elapid Walterinnesia aegyptia, all from Israel, show a similar ld50 of 250-300 μg/kg body weight when injected intravenously into mice. The venoms of the viperid Aspis cerastes from Israel and the elapid Naja nigricollis from East Africa are less toxic and have an ld50 of 600 and 1200 μg/kg respectively. Compared with mice on a weight basis, V. palaestinae, Natrix tessellata (a non-venomous water snake) and the mammal Herpestes ichneumon (mongoose) are highly resistant to V. palaestinae venom; Mesocricetus auratus (hamster) also shows some degree of tolerance. The sera from these and other animals were tested for their capacity to neutralize this venom in vitro. The snake sera, which represented the families Elapidae, Viperidae, Crotalidae and Colubridae, all neutralized V. palaestinae venom, whereas sera from two lizards (Uromastix aegyptius and Agama stellio) failed to do so. Of the mammalian sera tested, neutralization was clearly demonstrated with hamster serum and to a lesser degree with hedgehog (Erinaceus europeus) serum, but not with human serum, cat serum or rabbit serum, and even serum from the highly resistant mongoose failed to show any neutralization. In tests involving these sera and the 6 venoms listed above, the Viperidae venoms were neutralized by sera from snakes of the same family, by W. aegyptia serum, and, with the exception of E. colorata venom, by N. nigricollis serum. The neutralization by sera from non-venomous snakes was slightly less complete, and sera from the mongoose and hamster failed to neutralize in some cases. N. nigricollis venom could be neutralized by the homologous serum and by W. aegyptia serum, but not by any others. All test sera, including the homologous serum, were without effect on W. aegyptia venom. Despite the lack of neutralizing capacity of its serum, W. aegyptia is highly resistant to its own venom and the mongoose to both of the elapid venoms tested. This resistance is apparently not due to humoral factors.