Whole-body and tissue retention of the radionuclide, gallium-67 (${}^{67}{\rm Ga}$), was determined in adult${\rm C}3{\rm BF}_{1}$ mice after acute whole-body x-irradiation. A significant decrease in whole-body retention occurred after an exposure to 75 R. Decreased retention in soft tissues was a transitory effect and recovery of the ability to concentrate${}^{67}{\rm Ga}$ was dose dependent within the range of 100- to 950-R exposures. "Recovery" occurred at Day 3 after 100 R, and between Days 6 and 8 after 450 and 950 R. The radiation effect of decreased retention was most severe when the radionuclide was administered 24 hr after irradiation. In contrast to decreased uptake in soft tissues,${}^{67}{\rm Ga}$ concentration in whole femur increased after irradiation. Blood levels decreased rapidly in irradiated mice (${}^{67}{\rm Ga}$ injected 24 hr after 450 R) between 1 and 2 hr after administration of the radionuclide. Two hours after injection, its concentration was approximately 15% of that in nonirradiated, control animals. We conclude that decreased${}^{67}{\rm Ga}$ retention in mice after x-irradiation is in part due to an effect on a relatively radiosensitive, as yet unidentified blood or serum component that normally binds the radioisotope. Other as yet unidentified factors may also be operative, however.

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