Using several kinds of respiratory enzyme inhibitors, the author studied the effect of the inhibitors on infection of rickettsia to mouse. The investigations were carried out in vitro and in vivo and the observations were made from day to day. Moreover, the characters of rickettsia were investigated by the re-inoculation to normal mouse from the culture. The following results were obtained. 1) The infection of rickettsia were found to be suppressed by previous in vitro treatment of rickettsia with any enzyme inhibitors tested, but the suppression effect was varied by hind of inhibitors, its concentration, duration of its action and the concentration of rickettsial suspension. The infection capacity of enzyme inhibitor treated-rickettsia showed a decrease in cases of 5 hours treatment of 10-20 mg malonate, of 1.0 mg monojodo acetic acid or of 1.0 mg fluoride. However, the affection of these inhibitors was vary specific, and found to have caused merely a reduction of infection capacity or a suppression of infection instead of to caused an inactivation of rickettsia. 2) On the contrary the inhibitors did not show an inhibitory effect to the infection of the rickettsia, if the inhibitors were administrated to the mouse to be tested before the attack of rickettsia. Accordingly a fairly difinite infection and the successive multiplication of rickettsia were observed on the previousely inhibitor-treated mouse; upon rare occasions the infection was found to be severe on these treated mouse. 3) The effect of inhibitors also studied by obeorving properties of re-inoculated rickettsia to normal mouse, the rickettsia was taken from the culture on the previously inhibitor-treated mouse. The multiplication of re-inoculated rickettsia was measured by means of LD(50) curve, and was demonstrated that the appearance of symptomes and the days to death after the infection was somewhat delayed on the treated animal compared to the control; but the occurences were almost in the same way as in the control except a short delay of the time. Hence, the affected metabolism of host cell and metabolic system of animal required to multiplication of the rickettsia were supposed to show so much definite effect to the suppression of infection.