In order to ascertain the distribution of the bacteria introduced into an animal body by tracing radioactivity of radioisotope labeled organism, the author studied the properties of P(32) labeled Sal. 57 S and the distribution of radioactivity in mouse that imply the presence of the organism. The following results were obtained. 1) To obtain an appropriate organism for the investigation, the author observed the stability of bound radioactivity on the cells that were obtained by three different labeling methods. i.e. on resting cells, on 3 hrs cultured cells and on 18 hrs cultured cells respectively. Concerning to dissociation of radioctivity from labeled cells by repeated washing or incubation of the cells, it was noticed in vitro that the labeling on resting cells was most labile, then came next the labeling on 3 hrs cultured cells, but that on 18 hrs cultured cells was most stable. Moreover, the study of. P(32) incorporation into phosphorus fructions of the cells by Schneider's method showed the marked incorporation of P(32) into nucleic acid and protein fractions by the labeling on 18 hrs cultured cells compaired with other labelings. 2) The radioactivity of labeled cells was observed to be stable in vitro compaired with living cells, when the labeled cell was killed by application of heat. 3) From the observation of interrelation between the presence of living cells and the radioactivity in the body of mouse, it could be said that the presence of bacteria could be traced out by detection of radioactivity within 6 hrs after injection and that the situation became to be disturbed beyond that time possibly owing to the dissociation of P(32) from the organisms caused by metabolism and divisions of the cells. While in the case of killed organism the situation was remained undisturbed for a fairly long time than in the case of living cells for absence of metabolism and division of the cells. 4) When the labeled organism was injected intraperitoneally into mouse, it was observed the marked accumulation of radioactivity on spleen and liver of the animal 2-3 hrs after the injection. In the case of intravenous injection, a conspecious accumulation was occured even 1 hr after the injection. However, by subcutaneous injection the radioactivity did not spread out into the body and ramained the site of injection for a fairly long time. 5) In the study on a group of previously immunized mouse by a intraperitoneal in jection of killed organisms, a wide distribution of radioactivity into the body of animals was inhibited by the intraperitoneal challenge of the labeled cells. While, the distribution was almost the same as the control group excepting slightly increased accumulation of radioactivity into the spleen by the intravenous challenge of the labeled cells.