For the study and quantitation of the action of antibiotics on Rickettsia, a slide chamber method of culturing chick embryo cells was developed, and the technique used to plaque Rickettsia was modified. These methods permitted the detection of morphologic changes induced by antibiotics and the determination of minimal inhibitory concentrations and rates of killing of rickettsiae. With Rickettsia prowazeki the minimal inhibitory concentrations were 1.0, 0.1, 0.06, and 0.008 μg/ml for chloramphenicol, doxycycline, erythromycin, and rifampin, respectively. The in vitro actions of doxycycline and minocycline were indistinguishable from that of tetracycline. Doxycycline, rifampin, and erythromycin killed rickettsiae at clinically attainable concentrations, but the rates of action were too slow for eradication of the organisms from the human host under conditions usually associated with the treatment of typhus fever.