1. The susceptibility of mice of the RF strain to radiation-induced myeloid leukemia and lymphomas was studied under a variety of experimental conditions.

  2. Although the rate of induction of both diseases varied in relation to the dose of radiation, the relation between disease incidence and dose was not the same for both neoplasms; neither was it linear over the dosage range 0–500 r.

  3. On fractionation of a given dose of radiation, the rate of induction of leukemia varied depending on the total dose, dose per exposure, interval between exposures, and hematologic type of leukemia produced.

  4. Shielding of the pelvis and lower extremities during irradiation of the remainder of the body greatly inhibited the induction of myeloid leukemia as well as of lymphomas.

  5. Susceptibility to lymphoma induction, which is higher in females than in males, was decreased by ovariectomy but was not affected by orchidectomy. Susceptibility to the induction of myeloid leukemia, which is higher in males than in females, was not significantly affected by gonadectomy in either sex. The sex differences in the susceptibility to induction of lymphomas and myeloid leukemia persisted even after removal of the gonads.

  6. Removal of the spleen before irradiation greatly lowered susceptibility to the induction of myeloid leukemia but did not influence the induction of lymphomas.

  7. Newborn mice were resistant to the induction of myeloid leukemia despite susceptibility to lymphoma formation, whereas the reverse was true of mice irradiated at 6 months of age.

  8. Removal of the thymus before irradiation, which eliminated the induction of thymic lymphomas, permitted the induction of lymphomas in extrathymic lymphoid tissues but did not influence the formation of myeloid leukemia.

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