With the routine simple method devised in the Hiraki Clinic of Internal Medicine the author studied the motility of platelets (peripheral blood) in normal persons and obtained the following results. 1) This simple method of tissue culture makes it possible to observe in detail the movement of platelets for a longer period of time than with conventional method of the droplet specimens or living stretch specimens and also it makes possible to grasp the pattern of platelet movement more like in vivo. 2) The average of longer diameters of living platelets is 3.487 μ and that of shorter diameters 2.406 μ. 3) The author classifed the patterns of platelet movement into 4 types and 7 subtypes; namely, type Ⅰ, the rotating movement; type Ⅱ, deformation of cell; type Ⅲ, trembling movement somewhat like Brown's movement; and type Ⅳ, stretching entire body. 4) Platelets showing such different type of the movement have accordingly different grade of the functions; and the platelets having patterns of the type Ⅰ and Ⅱ are more numerous in the early stage of culture whereas they decrease in number with the lapse of time. On the contrary those platelets possessing the movement pattern of the types Ⅲ and Ⅳ are fewer in the early stage but increase along with the lapse of time; and the function of the former is more active than that of the latter.