The author investigated the occurrence of the skin reaction of rabbit of inoculation of the rickettsial material prepared by the treatment of rickettsia-infected organ emulsion with alumina gel. At the same time, by this skin reaction the infection titer of rickettsia was determined. The results are as follows 1. The alumina geltreated material causes redness in twelve hours after the inoculation, but induration and ulcer formation the most remarkable signs, appear from fourty-eight to seventy-two hours later. After ninety-six hours, both of these redness and induration gradually disappear. 2. The result of the skin reaction by alumina geltreated material should be judged from fourty-eight to seventy-two hours after the inoculation. It is valid to conclude that the criterion of the significant skin reaction is induration and the minimal skin dosis is the one corresponding to the maximal dilution which is much enough to cause this induration on the inoculated part. 3. Regardless of the sorts of strains, the intracutaneous inoculation of living rickettsia causes redness twelve hours after inoculation, of which, however, the duration is short. By the alumina gel treatment, killed material never acquires the ability to cause skin reaction, irrespective of the sorts of strains. 4. The ability of alumina gel treated rickettsial material to cause the significant skin reaction, is concerned with the degree of its dilution and its virulence and not with the sorts of strains.