By means of tissue culture the author studied the carbon particle phagocytosis of monocytes, histiocytes, transformed ascitic phagocytes and the new born cells under stimulation, using of mice, and obtained the following results: 1. Of ascitic cells those presenting the shape like fibroblasts or histiocytes are ascitic phagocytes merely showing one phase of transformations, and the phagocytosis capacity and other cell-functions of these cells are exactly the same as those of the ascitic phagocytes. 2. The usual ascitic phagocytes are the cells possessing different characteristics from the original cells (new born cells) due to the specific circumstances. 3. From the standpoint of phagocytosis capacity the ascitic phagocytes are fundamentally the same as monocytes, but they differ in many points from histiocytes. 4. From these results it is assumed that the ascitic phagocytes are not histiocytes but they are cells closely related to monocytes.