For the purpose of observing the influences of the spleen on the production of the platelet in bone marrow the author performed the bone-marrow tissue culture (cover-slip method) of guinea pigs and observed the function of megakaryocytes, and obtained the follo wing results. 1. In the bone marrow tissue culture of the guinea pigs transplanted with homoioplastic spleen conducted from time to time to the megakaryocyte function, when compared with that of the control group transplanted with normal spleen, shows a marked decline transiently about 3 to 5 days after the operation, but by 20th to 30th day it shows hardly any significant difference from that of the control. 2. In the bone-marrow tissue culture of normal guinea pigs added with Ringer's solution of spleen extract of normal guinea pigs, the extract at a higher concentration brings about a fall in the megakaryocyte function whereas on the contrary at a higher concentration it accelerates the function. 3. From these results it seems that the spleen of normal guinea pigs with the aid of some fluid agent acts on the thrombopoiesis of the bone marrow interchangeably either suppressingly or acceleratingly, but from the physiological aspect the suppressing action seems to be slightly stronger.