In various experimental infections inthe mouse, the single doses of chlortetracycline were determined which, when given simultaneously with the infection, prevent death of the animals. Serum concentrations of the antibiotic were measured in mice treated with 80–100% effective doses. During the initial period of infection, which is characterized by insufficient defence (low resistance period), these concentrations are invariably above or approximately equal to the minimum bacteriostatic concentration established in vitro. Hence, it is concluded that in animal experiments the dosage is dependent not only on the in vitro sensitivity of the infectious agent but also on the length of the LRP, during which the micro-organisms have to be controlled by a bacteriostatic agen