Antidysenteric serum can be safely prepared in the horse by the method of three successive intravenous injections of living cultures or toxin with intervening rest periods of seven days. When this method of immunization is employed, the specific antibodies responsible for agglutination and protection appear early and rise rapidly. By inoculating alternately living dysentery bacilli belonging to the Shiga and Flexner groups a polyvalent serum of high titer may be prepared. A polyvalent serum so produced should be suitable for the therapeutic treatment of acute bacillary dysentery, irrespective of the particular strain or strains of the dysentery bacillus causing the infection. An effective antidysentery serum suitable for therapeutic employment in man can be prepared in the horse in about ten weeks.