With model experiments we endeavoured to further clinical experience in that infection of patients in a state of immunodepression may be treated more efficiently with bactericidal than with bacteriostatic antibiotic drugs. In cases of lethal infection due to Staphylococcus aureus, we studied the behaviour of animals with intact or immunosuppressed immune systems under bactericidal and bacteriostatic antibiotic therapy. As index we used the mortality rate and the possibility of reculturing the infectious strain. In our experiments we studied the effect of diabetes mellitus and senescence as influencing factors on the course of infection. It was indicated that in diabetic model experiments the effect of bactericidal antibiotic drugs was more advantageous, both in reduced mortality and lower possibility of reculture.