In an attempt to study the relationship between the megakaryocyte function and the spleen in experimental thrombocytopenia, the author performed the tissue culture of the bone marrow and the spleen of normal guinea pigs and also the bone-barrow tissue culture of normal guine pigs side by side along with the spleen of the guinea pigs injected with anti-platelet serum on the same slide glass, each by a simple culture method; and observed the number and functions of megakaryocytes appearing in the growth area and the wandering velocity of neutrophils. 1. In the case of the bone-marrow tissue of normal guinea pigs cultured side by side with normal spleen on the same slide, megakaryocytes appering in the growth area was somewhat less in number and their functions were also lower than in the case of the bone-marrow tissue cultuse alone. However, there was no significant difference in the wandering velocity of neutrophils. 2. When the spleen of normal guinea pigs given anti-platelet serum was cultured side by side with bone marrow of normal guinea pigs, no significant difference in number of megakaryocytes appearing could be recognized as compared with the case with the normal spleen, but the functions were markedly decreased. The wandering velocity of neutrophils in both of these cultures was Identically the same. From these findings it has been found that in the spleen of the guinea pigs injected with anti-platelet serum there exists a factor which acts directly on megakaryocytes of the normal guinea pigs as to inhibit their platelet production.