As to comprehend the infectious disease, it seems to be very important to elucidate the interrelationship between the causable pathogenic microorganism and the host. In order to know the patho-biochemical changes of the cells of infected animal, the author carried out the investigation, in which the phosphorus metabolism were studied on the liver and the spleen of the infected mouse at day's interval from the infection. Sal. typhi murium, Rickettsia tsutsugamushi and Ectromeria virus were used at the pathogenic microorganisms for this study. The phosphorus metabolism was measured by means of P(32) incorporation into each phosphorus fraction of the organs of the animal tested that were fractionated by Schneider's method and was considered comparing with the changes found on the histology. The following results were obtained. 1) Compared with the phosphorus metabolism of each phosphorus fraction on the liver and the spleen of the rickettsia or virus-infected mouse, these metabolism on that of Sal. typhi murium-infected mouse were found to be highly accelerated at the 3 rd or the 5 th days of infection. 2) In the case of the phosphorus metabolism on the liver and the spleen of R. tsutsugamushi-infected mouse, the metabolism of the phosphorus fractions except nucleic acid P were found to be decreased at the 3 rd and 5 th days of infection, but were slightly accelerated at the 7 th day. However, the metabolisms of nucleic acid P were found to be accelerated throughout all the examinations, especially at the 3 rd and 5 th days. 3) On the Ectomeria virus-infected mouse the phosphorus metabolism was measured at the 2 nd and the 4 th days of infection. The metabolism of nucleic acid P fraction showed specifically very promnient acceleration, but that of the other fractions did not accelerated except only slight acceleration.