IN inoculation against typhoid fever, dead typhoid bacilli are injected subcutaneously or into a muscle. Inoculation against plague consists in the injection of an emulsion of dead plague bacilli artificially grown in broth. These are prophylactic measures for the protection of persons who are likely to be exposed to infection. Hydrophobia vaccine, used in order to prevent the development of the disease in persons who have been bitten by a rabid animal, is prepared according to a definite system from the spinal cords of rabbits inoculated with the disease. Anti-diphtheritic serum, used in the treatment of patients actually suffering from diphtheria, is the blood-serum of a horse which has had diphtheria toxin (the broth in which diphtheria bacilli have been grown, and from which the bacilli have been filtered off) repeatedly injected into it.