=General Method.=--We start this work with certain hogs that are immune usually because they have passed through an outbreak. It has been shown that when such immune hogs are treated with large injections of virulent blood under the skin or into a vein, that they do not usually become sick, but their own blood develops a peculiar property that gives protection to other hogs that are naturally susceptible. When the blood or rather blood serum from this specially treated immune hog is injected into the bodies of healthy susceptible hogs, the latter becomes likewise immune, but the immunity so gained lasts only a short time, possibly four to six weeks, and is then gradually lost. If we give a small injection of virulent blood at the same time, or soon after the immunizing serum is given, then the treated hog becomes immune for a long period, perhaps for life. =The Serum Hog.=--The specially immunized hog which produces this immunizing serum is known as a hyperimmune, and to save words will be hereafter mentioned as such. The simply immune hog may be prepared for producing serum in either one of three ways. (1) By three rapidly increasing doses of virulent blood serum injected under the skin at intervals of seven to ten days; (2) by one enormously large injection of virulent serum under the skin; (3) by injecting virulent blood in smaller doses directly into the blood circulation. In this work an ordinary immune hog weighing 100 pounds is given a quart of very virulent blood, a teaspoon of which similarly injected would kill a hog that was not immune. In other words the immune, and especially the hyperimmune hog, have developed certain properties in their blood antagonistic to hog cholera virus. =Vaccination.=--We have two possible methods of vaccinating or immunizing susceptible hogs (a) Serum only. This is by the injection under the skin of serum from the body of a hyperimmune hog and gives immediate but temporary immunity lasting, as already stated, several weeks. If this animal, during the period of immunity, is exposed to natural infection, he becomes protected for a very long period, perhaps for life. (b) Simultaneous. The second method of vaccination consists of injecting immunizing blood serum into one thigh and a small amount of disease-producing serum at the same time, or soon after, into the other thigh, thus giving the animal the cholera and a cure for it at the same time. If the immunizing serum is potent and the virulent serum is really virulent, then the animal so treated becomes permanently immune.