The sternal bone marrow tissues were cultured from the patients after admission in our clinic, and special care was given to the function of megakaryocytes before and after splenectomy as well as after injection of ACTH. The following results and conclusions were obtained. 1) The author has studied 8 cases of chronic I.T.P., in which 2 cases were cured by injection of ACTH and splenectomy, and another 3 cases by splenectomy only. In total, 5 cases were cured completely. 2) Most of the megakaryocytes of patients had neither motility nor tentacle-like filaments in spite of their maturity, and thus showed severe bypofunction. But remarkable recovery of the function took place soon after the splenectomy, and after injection of ACTH the author could also recognize the recovery of the function to some extent. 3) It is concluded that the genesis of chronic idiopathic thrombocytopenic purpur mainly exists in the hypofunction of megakaryocytes, and at least in 5 cases in which splenectomy was performed, the hypofunction was caused by the effect of the splenogenous toxic substance before-mentioned, the platelet reducing factor.