3imbesteae the immunological response of the host may be restored with early rejection of a noncompatible graft. At higher dose levels the graft may develop a lethal syndrome characterized by skin lesions, gastrointestinal dvsfunction, lymphoid tissue atrophy and generalized wasting away of tissues (Congden). This reaction is frequently termed ‘secondary disease,” and is believed to be chiefly the result of a reverse iminunological reaction, Le., ia ATOMIC MEDICINE Bate 226 reaction of the grafted tissue against that of the host. The reaction is po- tentiated if even small amounts of lymphocytic cells are transfused with the marrow. The secondary disease has becn demonstrated in several animal spevies including mice, rats, guinea pigs, hamsters, rabbits, monkeys and dogs. There appears to be little question that an individuals’ own bone marrow, or that from an identical twin will ‘ttake” if infused into the irradiated human being and may be life-saving. Long-term survival of antigenctically nonidentical (homologous) marrow has been reported in a human being who had received chemotherapy for a blood dyscrasia (Beilby}. Mathé has reported a temporary “take” of homologous marrow fee, reactor accident, and has reported successful transplantation and ‘‘seeondary discase” in leukemic children given high doses of whole bodyradiation followed by infusion of homologous marrow. The procedure has not been curative when applied to individuals with leukemia. However, radiation apparently has allowed successful transplantation of homologous kidneys in the humanbeing (Merrill e¢ al.) and more practical applications may be expected in the rapidly developing field of tissue homotransplantation (see Report of Tissue Homotransplantation Conference). oilse given to several individuals who received high doses of radiation in a te SS frag dog ang encameene tamnngen et Thus it must be accepted that transfer of viable cells with the potential of restoring hematopoietic tissues (and the immune response) does occur. However, the identity of the stemcell or cells transferred remains unknown. It must be recalled that with shielding of the spleen or marrow, apparently cells capable of stem cell activity are carried via the blood stream and initiate repopulation of the depleted marrow areas. Injected peripheral blood cells and cells from peritoneal washings wil proliferate as will bone narrowin the irradiated host. It is known that a small percentage of cells normally present in the peripheral blood are capable of DNA synthesis and thus presumably of proliferation. Evidence has been presented indicating that these may not be the same cells that give rise to mitotie figures when normal peripheral blood is cultured under appropriate conditions. The problem appears to blend into, and may answerat least in part the more general old hematological problem of the origin of extramedullary hematopoiesis. Is it autochthonous or metastatic? Proof of cell transplantation does not rule out or exclude completely a humoral contribution with stimulus for autochthonous growth. However, it does seem clear oor o f., % :