By radiosensitive tumors of the blood-forming organs we refer to those tumors which are derived from primitive cells of the lymph nodes, bone marrow and spleen whose normal maturation gives rise to adult forms seen in normal blood; namely, lymphocytes and cells of the myelocytic series (polymorphonuclear neutrophils, eosinophils and basophils). Tumors derived from the primitive endothelial cells of the lymph nodes and marrow, whose normal maturation produces the large mononuclear cell seen in normal blood, are so rare that they have not been included in this paper. Following the terminology of Mallory, we have chosen the terms "lymphoblastoma" and "myeloblastoma" to designate these tumors, as the unqualified use of various terms, such as "malignant lymphoma," "lymphosarcoma," "pseudoleukemia," "splenomyelogenous leukemia," and the like, have in the past only added to the confusion in regard to the myeloblastoma, as this tumor usually expresses itself in a characteristic clinical picture; i. e.,