Abstract: The invention relates to the preparation of novel immunogenic agents and compositions of vaccines to combat salmonellosis caused by Salmonella abortus ovis and Salmonella typhimurium.The preparation is that of the mutant strains of Salmonella abortus ovis and comprises:(a) the seeding and culture of bacteria from a virulent wild strain, on a medium containing an antibiotic enabling the development of "resistant" mutants;(b) the culture by replication of the resistant bacteria on a medium free of antibiotic to determine those which are "dependent";(c) the seeding of the culture of the dependent bacteria derived from (b) on medium free of antibiotic until the appearance of reverse colonies.The immunogen agents obtained are useful for the preparation of vaccines.