In the study of cells in the pleural fluid of the mouse and rabbit, cspccialfy phagocytes, the author obtained the following; results: 1. As in the case of ascites phagocytes are decidedly numerous in the pleural fluid of the mouse and rabbit. 2. The morphology of the phagocytes found in the pleural fluid is almost identical with that of pagocvtes in the ascites. 3. The motility, transformation, and the course of degeneration of phagocytes in the pleural fluid more or less coincide with those of phagocytes in the ascites. 4. From these findings it seems rational to assume that the phagocytes in the pleural fluid are the same kind of cells as phagocytes in the ascites, and consequently, belong to the same cell series as monocytes. 5. Phagocytes in the pleural fluid, as well as those in the ascites, change their own shapes to fibroblast-like or histiocyte-like figures in the medium of tissue culture, but essentially they do not lose their original character as phagocytes. However, this transformation is dependent upon various condition of the culture.