Comparative studies were carried on the normal ascitic cells, especially on phagocytes that occupy the major portion of ascitic cells, aspirated from mammalians such as mice, rats, rabbits, dogs, cats, and human as well as from chicken by the pressure method with a phase contrast microscope and also with moving pictures of these various cells. As the results the following conclusions were arrived at: 1. The majority of mouse phagocytes have the characteristics similar to those of monocytes and histiocytes. 2. In the higher animals such as rats and rabbits a greater portion of them show flagellar pseudopodia and the aggregating tendency of highly reflactile granules and the distribution of mitochondria along the groove of the nucleus. And their nuclear membrane is thin and the nuclei are solid with a complicated shape, and the nuclear substance is soft but without any nucleoles. All of these findings become quite similar to those of monocytes, and in dogs and cats they can hardly be distinguished from monocytes. In the case of normal human phagocytes most of them show degeneration but even these cells reveal the characteristics very close to those of monocytes but far different from those of histiocytes. Even in the chicken phagocytes an unexpectedly large portion of them possess marked characteristics of monocytes. 3. As for the movement of these cells it is minimal, differing from blood monocytes this point, but it seems that this is due to the adaptation peculiar to the circumstances as ascites. From these findings it can be concluded that the ascitic cells possess characteristics far different from those of histiocytes but extremely similar to those of monocytes.