The use of a vaccine is based on two fundamental principles. The first of these is that the cell introduced must not be in a condition to cause serious injury to the animal by its multiplication and consequent elaboration of injurious substances. The second is that, on the other hand, it must contain antigens in such condition that they will act as stimuli to the body cells to produce the necessary antibodies, whether these be opsonins, bactericidal substances, or anti-endotoxins. In the introduction of living organisms there is always more or less risk of the organism not being sufficiently attenuated and hence of the possibility of its producing too severe an infection. In using killed cultures, great care must be exercised in destroying the organisms, _so that the antigens are not at the same time rendered inactive_. Hence in the preparation of bacterial vaccines by Wright’s method the _temperature and the length of time used to kill the bacteria are most important factors_. This method is in general to grow the organisms on an agar medium, rub off the culture and emulsify in sterile normal salt solution (0.85 per cent. NaCl). The number of bacteria per cc. is determined by staining a slide made from a small volume of the emulsion mixed with an equal volume of human blood drawn from the finger and counting the relative number of bacteria and of red blood corpuscles. Since the corpuscles are normally 5,000,000 per c.mm., a simple calculation gives the number of bacteria. The emulsion of bacteria is then diluted so that a certain number of millions shall be contained in each cc., “standardized” as it is called, then heated to the proper temperature for the necessary time and it is ready for use. A preservative, as 0.5 per cent. phenol, tricresol, etc., is added unless the vaccine is to be used up at once. The amounts of culture, salt solution, etc., vary with the purpose for which the vaccine is to be used, from one or two agar slant cultures and a few cc. of solution, when a single animal is to be treated, to bulk agar cultures and liters of solution as in preparing antityphoid vaccine on a large scale. Agar surface cultures are used so that there will be as little admixture of foreign protein as possible (see Anaphylaxis, p. 289 _et seq._). Normal saline solution is isotonic with the body cells and hence is employed as the vehicle.