The mice, pre-immunized by the subcutaneus inoculation of Rickettsia tsutsugamushi, were intraabdominally reinoculated with the same strain. The author tried to investigate the resistance against reinoculation by observing the proliferation of the rickettsia, which was estimated by the infections titer. of the animal livers. The results are as follows; 1) The mice, reinoculated on the second and seventh day after the first inoculation, showed a fairly high resistance of the same degree. The highest resistance was observed on the fifteenth day, and on the thirty-second day the resistance somewhat falled, but on the fifty-fourth day it rised again. At this case, the neutralization antibodies in mice were observed to fairly high on the thirty-second day, and increased more on the fifty-fourth day. 2) The same experiment was carried out on the forty-seventh. fifty-second, sixtieth, seventy-second and ninety-nineth day after the first inoculation. The resistance against intraabdominal reinoculation was the highest on the forty-seventh day, the next on the fifty-second day and then falled to a certain degree, which persisted for pretty long time.