For the purpose of ascertaining the existence of platelet reducing factor in several materials obtained from 5 patients with I.T.P., those whom the author has studied, these materials were injected subcutaneously into rabbits. Moreover, additional experiments were made to investigate the effect of this factor upon the bone marrow. The results are as follows: 1) The patients' materials caused a marked reduction of more than 45% in the number cf platelets in the circulating blood when injected into rabbits. But materials obtained by same procedure from control human beings caused less than 30% reduction as compared with those before injection. 2) The existence of platelet reducing factor was found in patients' sera, urine, cerebrospinal liquor and spleen extracts. 3) 2 cases out of 5 were completely cured by splenectomy, and after operation this factor could not be found in their sera. 4) This factor is thermolabile and soluble in water. 5) Through the bone marrow streaming examination, this factor was found to act directly upon the bone marrow. 6) It is concluded that the continuous reduction in the number of platelets in these patients is the result of the inhibiting effect upon bone marrow by this factor produced from the spleen.