The hematologic effects of Ga72 administered intravenously to patients with diffuse bony metastases or primary bone malignancy were studied.

1. The effects of this internally administered isotope were, in general, similar to those resulting from total-body external radiation. The degree of effect was greater than that anticipated from estimates of the total-body dose received from the isotope. Localization in the bone may have been responsible for this finding.

2. The depth and duration of platelet count depression probably represented the best indices of the degree of marrow damage after radiation, and may be indicative also of the total damage sustained by the marrow from previous exposure.

3. The effects of large doses of radiation on the marrow appeared to be cumulative to a point of no return, beyond which regeneration may not be possible.

4. It was found that a total white count below 1,000 and a platelet count below 25,000 could be tolerated for weeks without infection or gross bleeding and with ultimate recovery. Therapy because of these findings alone did not appear to be indicated.

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