The vaccine is usually prepared from a fresh twenty-four-hour growth of a pure culture of the micro-organism on an agar slant. The growth is scraped off and made into an emulsion with physiologic salt solution. The emulsion is then sterilized by heating at 6o° C. for one hour and afterward is further diluted. This is done so that I c.c. will represent approximately the dose to be given. This dilution will vary, as the number of bacteria to a dose varies according to the organism used. Culture-tubes are inoculated with the vaccine and incubated for twenty-four hours at 370 C. to make sure that the sterilization was complete. To preserve the bacterin, 0.5 per cent, of carbolic acid or trikresol is used.

The injections should always be given subcutaneously. Usually three or four injections are given at intervals of from five to ten days, as in this way an immunity of much higher grade and longer duration is obtained. In most instances the acquired immunity lasts from two to five years and may be renewed.

Preventive inoculations with bacterial vaccines have been used extensively against typhoid fever, plague, and cholera.

The bacterins are used also as curative agents, but much care should be observed in giving the proper dosage. A minimum amount should be given, and if it creates no unfavorable reaction, a larger amount should be given subsequently.