The minimum inhibitory concentration of bactericidal as opposed to bacteriostatic agents does not necessarily have to be maintained in the blood for the full duration of the low resistance period (LRP). Even brief exposure to concentrations which are bactericidal or high enough to induce bacteriopause can effectively suppress the infection. The longer the LRP, the more sustained must be the antibacterial effect. The relations between these factors are examined with reference to the effect of single doses of rifampicin and cephaloridine in a series of experimental infections in the mouse.