Having been tempted by the fact that the difference between histiocytes and monocytes is not only marked in their morphology but also most prominent in their motility, the author present in this paper the results of comparative studies on the true entity of phagocytes in the ascites, namely, on the question whether they should belong to the histiocyte series or to the monocyte series, with a special emphasis on the motility: 1. Phagocytes under normal conditions as well as under stimulation possess the motility quite similar to that of monocytes. 2. When monocytes are cultured in the medium containing ascites, it has been confirmed that they are converted to cells somewhat like phagocytes in their behaviors and characteristics. 3. Moreover, since it has been recognized that the behaviors of subcutaneous histiocytes not only differ from those of phagocytes or monocytes but also the motility of the former is inferior to that of the latter two, and that the motility is not accelerated even under stimulation, the entity of phagocytes seems never to be the same with that of histiocytes but rather quite akin to monocytes. However, as the phagocytes do not possess the wandering capacity in the same marked degree as monocytes, a due reservation must be made before deciding them to belong to monocytes. Nevertheless, admitting that the origin (milk spots in the greater omentum) and peculiar circumstances of being contained in the ascites are responsible for this difference, it may not be so unreasonable to consider phagocytes to belong to the same cell series as monocytes.