1. Local Co60 teletherapy caused a reduction in leukocytes and lymphocytes in the peripheral blood.

2. The bone marrow demonstrated no morphologic change in nonirradiated control sites.

3. Local irradiation produced a pronounced and persistent hypoplasia in the treated sites during and after irradiation, with a great reduction in the numbers of megakaryocytes and precursors of red and white cells. During the period of greatest radiation effect the persistent cells were chiefly plasma cells, "mononuclear cells," and lymphocytes.

4. Even after "cancerocidal" radiotherapy, irradiated bone marrow shows some capacity to regenerate as evidenced by appearance of precursors of various cell series and their ability to incorporate tritium-labeled thymidine.

5. Hemosiderin increased in varying degrees in irradiated sites but showed no change in the control sites.

6. Satisfactory marrow samples can be aspirated from the pubic bone.

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