The effectiveness of five internal emitters in depressing blood leukocytes in young adult beagles was studied during the first year postinjection. The radionuclides,226 Ra,239 Pu,228 Th,228 Ra, and90 Sr, were given as a single intravenous injection, at 6-7 injection levels per radionuclide. The occurrence of dose-dependent depressions in leukocytes has been previously established. The greatest depression was found in polymorphonuclear leukocytes (pmns), monocytes, and eosinophils while lymphocytes were depressed to a lesser extent. The probit transform was used to linearly relate response to the logarithm of the injected activity. Injected activities required to produce a 50% leukocyte depression ( ED50) were computed for eight time periods. Toxicity indices were also computed for four radionuclides relative to226 Ra as a standard and converted to hematological RBE, again relative to226 Ra. For pmns and lymphocytes median values of RBE estimated over the eight time periods were:${}^{228}{\rm Th}\sim 4$;${}^{239}{\rm Pu}\sim 4$;${}^{228}{\rm Ra}\sim 1.5$, and${}^{90}{\rm Sr}\sim 0.5$. Alpha emitters depositing on bone surfaces were more effective in reducing leukocytes than those which are volume seekers. The beta emitter,90 Sr, was less effective than the alpha emitters rad for rad.

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