_Neurotoxin_ being the essentially active substance in venoms, and that to which the dangerous properties of poisonous snakes, as regards man and domestic animals, are especially due, it is the effects of this that it is most necessary to prevent. Consequently, the first quality that an antivenomous serum ought to exhibit, in order to be capable of being used in the therapeutics of poisoning, is the possession of an _antineurotoxic_ power as high as possible. This antineurotoxic power is easily obtained by employing cobra-venom for the fundamental immunisation of the horses destined for the production of the serum. _Antineurotoxic_ serum thus prepared shows itself perfectly capable of preventing all effects of intoxication from cobra-bites, which are much the most frequent in India. In the same way it shows itself quite sufficiently efficacious with regard to COLUBRINE and VIPERINE venoms, the neurotoxic activity of which may cause death. But it does not possess any preventive action upon the local effects of _hæmorrhagin_, to which the noxiousness of certain VIPERINE venoms--such as those of _Lachesis_--are almost exclusively due. In countries in which VIPERIDÆ are very common, we must therefore not confine ourselves to vaccinating the animals that produce serum solely against the _neurotoxin_ of cobra-venom, for instance; we must prepare these animals, after having immunised them to cobra-venom, by injecting them with progressively increasing doses of the various venoms derived from the snakes that are most frequently met with in the district. Nothing, moreover, is easier than to train animals vaccinated against cobra-venom to tolerate strong doses of the venoms of _Lachesis_, _Vipera russellii_, _Crotalus_, _Hoplocephalus_, or _Pseudechis_. In a few months we succeed in obtaining serums very active against these different venoms. Utilising the horse as producer of antitoxin, I have prepared by this method _polyvalent_ serums capable of preventing the local action of VIPERINE venoms, and of suppressing _in vitro_ their coagulant and proteolytic effects upon the blood.