Summary. Free cells in the pleural cavity of the mouse were studied by quantitative and qualitative morphological procedures.The size distribution curve of the cells, obtained by a Coulter counter and channelyzer, indicated that the pleural free cells were composed mainly of two populations, which were morphologically classified into small and large types. The small cells were about twice as numerous in females as in males, and the large type of cells exhibited no significant difference in number between the two sexes. Therefore, the total number of pleural cells was significantly greater in females than in males.The small pleural cells were similar in cytological features to the peritoneal mediumsized mononuclear cells described in previous papers (ABE et a!., 1979a, b) with the exception of their size. Typical small lymphocytes were also included among the small type of pleural cells, but they were very few in number. The large type pleural cells were macrophages. The pleural macrophages were somewhat variable in cell features, particularly in surface configuration, as compared with peritoneal macrophages.