Experiments were performed to test the hypothesis that rats could be made permanently anemic by repeated mixed gamma-neutron irradiations and that once the maintenance of normal circulatory red cell concentration was lost, the administration of exogenous erythropoietin could not restore the production of red cells to normal levels. Rats were exposed to nine periodic doses of 150 rads of mixed gamma-neutron radiation or to four periodic doses of 300 rads. Hematocrits and erythrocyte counts obtained for 100 days or more after the final radiation exposure showed a significant reduction in erythrocyte production. This permanent anemia was not ameliorated by the treatment with five daily doses of either 5 units or 25 units of erythropoietin. These findings appear to strengthen the hypothesis that the permanent anemia is caused by a reduced capability for cellular proliferation due to accumulation of residual injury in stem cells.

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