The auther carried out the experiments on the susceptibility of the mouse and embryonated egg to the strains of R. tsutsugamushi newly isolated in the various areas of Kagawa prefecture of japan. where "Umayado disease" and its suspected cases had been prevailing. The results are as follows: 1. The mouse shows greater susceptibility to the R. tsutsugamushi strains isolated in Kagawa prefecture than to the other reported ones. sometimes, however, it is not always infected according to the routs of inoculation'and, among various methods, intraperitoneal and intracranial inoculation are better methods becouse these take more stable infectious course, but it is necessary to change the methods according to the aim of experiment. 2. The mortality of the mice inoculated with this R. tsutsugamushi begins to rise from fifth day of inoculation, reachs the maximum on the twelveth or fourteenth day and then falls down. The rickettsiae, however, seems to stay in the bodies for a long time. 3. The susceptibility of the embyonated egg is very specific, namely the fixation and naturalization to it ability is different according to the sorts of strains.
In the case where many riekettsiae are observable in yolk sack, LD50 is proved to be very high.