Two radionuclides which localize in different body organs, may elicit antagonistic, synergistic, or directly additive biological effects when administered in combination. Graded doses (range, 1.4 to 5.3 mCi/kg body weight) of the bone seeker90 Sr were injected intraperitoneally into rats simultaneously with graded doses (range, 3.4 to 26.8 mCi/kg) of the rather diffusely localizing radionuclide137 Cs. Times to reach acute death in 50% of the animals ranged from 9 to 18 days. Radiation doses calculated for these injected mixtures ranged from 3900 to 11,000 rad to the skeleton from90 Sr and from 320 to 2200 rad to the whole body from137 Cs. There is some indication from comparing the time to acute death with the calculated radiation doses, that these radionuclides acting in combination give at least an additive effect for this endpoint (lethality).

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