On this occasion the authors examine how the hot-spring bathing influences on leucocytes and serum opsonin both separeted from living body, which in test tube dipped in the hot-spring bath after separeted. These results are as followed. 1) The bathing influences on the leucocytes, activating their function and promoting phagocytosis of leucocytes. 2) While, little influence upon serum opsonin. These results are in accord with general medical opinion and the increasing serum opsonin effected by bathing of living body, may be resulted from the protoplasm activation of the place of antibody production, and from the hypersecretion of opsonin in the blood stream. The opsonin, however, would not increase, when the serum put in the hot-spring bath, after separated from living body. The leucocytes separeted out of living body would be, by hot-spring bathing, activated and by the activation of these cells the leucocyte phagocytosis would be promoted.