An attempt has been made at the cytological study on normal ascites and thoracic fluid obtained by puncture from man and various animals, by counting number of cells in respective fluid, by preparing smeared-stained specimens, and by performing peroxidase reaction. The results are as follows: 1. As for the cell count in ascites, in mouse it was 74,100, in rat 67,221, in rabbit 1,795, in dog 3,040, and in man 1,161/㎣ , respectively; while the count in thoracic fluid, in mouse it was 68,337, in rat 75,560, in rabbit 1,555, it dog 3,177, and in man 1,163/㎣ , respectively. 2. As for the cell composition in every case phagocytes occupy the major portion as high as 80.6-92.5 per cent of the total cells, and the remainder consists of a few of lymphocytes. granulocytes, mast cells, and serosal cells. 3. By their sizes phagocytes may divided into a large type, intermediate type, and small type, and from smeared-stained specimens they show four types of stain picture, namely, monocyte-like, histiocyte-like, fibroblast-like, and lymphocyte-like pictures. However, in lower animals they resemble monocytes from the looks of their nuclear membrane and azure granules, and from the feature of nuclear meshwork and cell body they resemble histiocytes; and the higher the animals, the greater is the monocytic characteristic of these phagocytes. 4. In those mice, rats, dogs, cats, monkeys and man, whose peroxidase reaction of monocytes is positive, the same reaction of phagocytes (7.2% -23.8% ) is also positive; while on the other hand in those rabbits whose peroxidase reaction of monocytes is negative, the same reaction of phagocytes is negative. Moreover, the percentage of positive reaction in phagocytes is not in parallel with the percentage of granulocytes. Therefore, phagocytes can be adjudged to be peroxidase reaction positive cells possessing essentially the same characteristics as monocytes, and it is clear that these cells are a close relative of monocytes.