It has been remarked that oxidation destroys the toxins _in vitro_, and it has been thought that a process resembling disinfection may well take place within the tissues of the animal economy, but no decision has been arrived at regarding the possible mechanism of this action, which some attribute to the action of the oxidizing ferments of the organism, or to the action of certain special cells. According to Poehl, there is developed as destroyer a substance possessing energetic oxidizing properties, which he has isolated and named _spermine_, and which is found in most of the organic fluids and particularly in the leucocytes, the special rôle of which we will presently study. There develops still another cause of elimination, or, to be more exact, of the neutralization of the toxic principles in defense of the organism against the toxins, and that is the formation of _antitoxins_. It is well known that the term _virus_ has been reserved to designate physiological liquids which were characterized, when first they were known, by their property of transmitting to an organism certain functional affections, but the true character of which is to expend their toxicity upon the microbes which occur and are reproduced in the organism, or upon the organized plastidulary granulations, as in the case of the rabic virus, the special microbe of which has not as yet been isolated. Pasteur, when studying rabies, found that the brain and spinal marrow of rabid animals contained the pure rabic virus in considerable quantity, and that every particle of the marrow was capable of imparting rabies to a perfectly healthy dog. After having ascertained this fact, he found that he could _attenuate the action of the virus_, either by passing the virus through certain animal organisms, such as the monkey or rabbit, by gently heating, or even by allowing it to oxidize and partially dry in the air, or else by submitting it to the action of antiseptics or alternating electric currents of very high tension. Experiments have shown that a deadly virus, attenuated by one of the means mentioned, may be injected, without danger of death, into the living animal; and what is still better, the animal thus treated acquires the power of resisting large doses of the virus, less and less attenuated, and that it is possible to reach a point where the animal economy may become habituated to very large doses of a highly virulent virus without the organism experiencing any visible illness--that is, the organism has been _vaccinated_ with regard to the particular virus. Experiments have shown that this property is not peculiar to microbial virus alone, but that it is common to the venoms the toxicity of which is essentially due to some toxins, with the exception of those agents noted.