With the purpose to see what role is played by spleen in experimental thrombocytopenic purpura the author injected anti-platelet serum into the splenectomized guinea pig and studied whether or not such splenectomy would affect the function of megakaryocytes by means of bone-marrow tissue culture. 1. When the guinea pig is subjected to splenectomy, thrombocytes in the peripheral blood tend to increase slightly in number. 2. When the bone-marrow of the splenectomized guinea pig is studied by bone-marrow tissue culture an increase of megakaryocytes in number can be observed but such an operation has no effect on the megakaryocyte function. 3. When the anti-platelet serum is injected into the splenectomized guinea pig, a remarkable decline in number of thrombocytes is recognized, but its degree and the decrease in the function of megakaryocytes are somewhat lesser than those observed in the case of the normal guinea pig injected with the same serum. 4. In the bone-marro tissue culture of the splenectomized guinea pig injected with anti-platelet serum, the number of appearing megakaryocytes somewhat increases and the megakaryocyte function slightly decreases as compared to those in the case of the splenectomized guinea pig without such an injection. 5. Judging from the above results, the spleen might slightly affect the decline in number of thrombocytes and the decreased function of megakaryocytes by anti-platelet serum.