_Immunization._ A number of different methods have been practiced of rendering animals refractory to the bacillus anthracis, but all are apparently based on the production in the system of defensive products, as the result of a nonlethal poisoning with anthrax toxins. It is true of anthrax as of many other infections that a first attack protects against a second. In all animals there is a certain measure of defensive power against the bacillus anthracis, amounting in some cases to virtual immunity and in others having a very little effect. The object in immunizing is to stimulate to the increase of these defensive products in quantity or power until an ordinary dose of the bacillus will fail to colonize the tissues or the blood. In considering this subject a clear distinction must be made between the simple bactericidal and the antidotal or antitoxic products found in the serum of immune animals and the toxins which are produced by the bacilli. The soluble antitoxic and bactericidal agents found in the serum of the immune, may be employed for therapeutic purposes to preserve life in an animal which has received a lethal dose of the bacillus anthracis, but as these are rapidly eliminated from the system, their protective power is very short lived, and if some bacilli survive the period of their presence and potency, or if they are introduced into the system later, the animal may fall a victim to anthrax as if no such protective agent had been used. Behring showed that the blood serum of the white rat proves fatal to the bacillus anthracis, but Metchinkoff pointed out later that it must be brought in contact with the bacillus in order to prove effective, whereas if the serum and bacillus were injected at different parts of the body no protection was obtained. The antidotal or bactericidal action of the serum of an immunized animal acts at once, whereas a permanent immunity cannot be established before about fifteen days. The serum of the immune animal contains the following elements antagonistic to anthrax: _Antitoxin_ or _leucomain_ which may be poisonous to the bacilli, or chemical antidotes to their products: _globulicidal principles_ which distort or disintegrate the blood globules and release their contents including the bactericidal nuclein, and probably others. All such agents when injected into the system are present only for a limited time and while they may be made subservient to a temporary immunity, they can give no permanent protection and must be considered mainly as therapeutic agents.