Using pleural fluid of mice and rabbits the basic-dye staining was carried out by means of tissue culture with a special emphasis on phagocytes, and these were compared with the phagocytes in ascites. The following are the results of the present study: 1. The basic type in the vital staining findings of the phagocytes in pleural fluid coicides exactly with those of the phagocytes in ascites. Furthermore, on taking other findings into consideration, it is certain that these two are identically the same kind of cells and they are near-relatives of monoctyes. 2. However, phagocytes in pleural fluid tend to show a more marked neutral-red rosette formation than the phagocytes in ascites, while on the contrary the degree of neutralred stain, the time retaining the stain and also the time retaining Janus-green stain are inferior to those of the phagocytes in ascites; and this phenomenon seems to be due to the immaturity of the phagocytes in the pleural fluid.