Survival of experimental animals after whole-body irradiation can be strikingly enhanced by different methods. Chemical agents such as glutathione (1), cysteine (2) and related SH-compounds, ,-mercaptoethylamine or Becaptan (3-6) and, S-0-Aminoethylisothiuronium Br HBr (7) have been recently shown to protect mice against early radiation deaths. Strong experimental evidence exists that survival time is also increased by spleen shielding (8) and by injection of homologous and heterologous bone marrow (9, 10). Radioprotection bears a close relationship to some facts reported by Kaplan in the pathogenesis of X-ray-induced lymphosarcoma in C57BL mice (11). Some measures that afford protection will inhibit the induction of lymphoid tumors. Similarly, this inhibition can be brought about by spleen shielding (12, 13) or by injecting homologous bone marrow cells suspension into the irradiated mice (14). Whether the chemical protective agents against early radiation deaths will also lessen the likelihood of induction and development of leukemia remains to be