1. With the high dosages described in this paper, cortisone, A.C.T.H., sodium salicylate and P.A.S., injected to rats immediately after the inoculation of S. typhimurium, increase the mortality rate. A similar result is obtained when some dosages of these drugs are administered during the days prior to bacterial infection. 2. Given in smaller dosages after the infection, Cortisone, A.C.T.H., sodium salicylate and P.A.S. seem to hinder the infectious process. Small amounts of cortisone given prior to bacterial inoculation increase mortality. 3. Administered after the infection of the rat by S. typhimurium, Cortisone, A.C.T.H. and perhaps also sodium salicylate associated with chloramphenicol, increase the action of this antibiotic. 4. Given at high dosages before bacterial inoculation, cortisone increases the mortality although chloramphenicol was given at the time of infection. In similar conditions, A.C.T.H. and sodium salicylate are without action. 5. The application of these findings to human medicine deserves further study. 6. A study of the metabolism of various corticosteroids in infected patients, treated or not with an antibiotic should be undertaken to ascertain further application of hormone therapy.