By observing monocytes and subcutaneous histiocytes in mice and rabbits, and by investigating still further phagocytes, the author discussed the similarity and dissimilarity of these cells; and also made a detailed study on essential charcteristics of phagocytes newly born in the ascites at the time of stimulation by checking periodical changes of smeared-stained specimen findings and cell counts and the peroxidase reaction; and obtained the following results: 1. Phagocytes, possessing as they do characteristics of both histiocytes and monocytes, can not conclusively be said to resemble one of the two as they themselves are. Morphological changes ocurring in phagocytes due to particular environments of ascites are responsible for this phenomenon. 2. Under a stimulation the cell count increases in ascites, and of them granulocytes and small phagocytes (newly born phagocytes) occupy a greater part, and also number of lymphocytes increases to a certain degree. 3. From findings on nuclear membrane, nuclear meshwork, chromatin, nuclear lobes, and vacuoles of the cell, new-born phagocytes present findings similar to those of immature monocytes and along with the lapse of time they present findings of mature monocytes by developing azure granules and pigmentation close to nuclear indentation; and still further vacuoles in the cell increase, and nuclear meshwork becoms irregular and the cell body grows cloudy, presenting findings similar to histiocytes, and then degenerate. Namely, phagocytes essentially possess the characteristic findings of monocytes. 4. On comparing the peroxidase reaction of phagocytes under the stimulation between that of mouse and rabbit, because the mouse with a low phagocytic power shows a higher positive percentage of the peroxidase reaction than in rabbit, peroxidase reaction granules of phagocytes are not all due to phagocytosis of neutrophils, and it has been reconfirmed the conclusion of Part 1 that in the case of mouse the peroxidase reaction is positive and in the case of rabbit negative. Although the positive rate of this reaction decreases as the cells degenerate and become aged, even then a higher positive rate is maintained in mouse than in rabbit. Therefore, this cell can be said to be positive cell. 5. From these results, it is concluded that phagocytes in the serous cavity are cells closely related to monocytes.