The author conducted comparative study under a fluorescence microscope between the ascites phagocytes on one hand and monocytes of blood and subcutaneous histiocytes on the other both from normal mice and stained by acridine orange, and further compared the phagocytes of the mice under stimulation and the subcutaneous histiocytes as well as the phagocytes at the time when the greater omentum is resected. In addition, the author ntilized the findings on monocytes in human monocytic leukemia as the reference; and obtained the following results. 1. In the case of active cells of monocytes in blood, the nucleus and cell body present green fluorescence, and the nuclear membrane and nuclear meshwork are indistinct. Reddish orange cytoplasmic granules do not tend to agglomerate but the majority of them are situtated around the nucleus. However, with the lapse of time both nuclear membrane and meshwork grow more distinct, and also the reddish orange cytoplasmic granules begin to agglomerate to a certain degree. 2. In the case of the subcutaneous histiocytes both under normal condition and at the time of stimulation the nuclear meshwork is coarse and the nuclear membrane is thick and clear-cut, and reddish orange cytoplasmic granules are great and almost uniform in size. Moreover, these granules are diffusely scattered but never tend to agglomerate. 3. In the ascites at the time of stimulation amall phagocytes are more numeroua, and these cells are poorly stained. However, the nuclei and cell bodies present a clear and deep green fluorescence, and reddish orange cytoplasmic granules do not necessarily tend to agglomerate, and their size is almoat uniform. With the lapse of time, both the nucleus and cell body are stained deep, also the cytoplasmic granules begin to agglomerate. 4. In the study of the senilization of phagocytes by inhibiting the cell regeneration by removing the greater omentum, the nuclei become green with yellowish tone and the reddish orange cytoplasmic granules, losing their agglomerating tendency, are scattered diffusely, but unlike subcutaneous histiocytes their size is extremely variable, and the cell body reveals a degenerative picture. 5. In the bone-marrow tissue culture of human monocytic leukemia by the simple method of our own device and in the medium of ascites, and vitally stained with acridine orange, the fluorescence picture of the monocytes at regeneration and senilization stages resembles qeite closely to the findings of human ascites phagocytes. 6. From these comparative study of fluorescence pictures of these various cells, it has been verified that ascites phagocytes unlike histiocytes are the cells quite closely related to monocytes.