Erythrocytes (RBC) from mice given phenylhydrazine are protective when injected into syngenic mice before X irradiation. The <tex-math>${\rm LD}_{50/30}$</tex-math> was increased about 90 R when these RBC (PhRBC) were given iv 1 day before exposure. PhRBC were not protective when given 4 hr after irradiation. Radioprotection by PhRBC did not appear to result from either an increase in the radioresistance (D0) of hemopoietic stem cells or an increase in number of stem cells at risk at the time of irradiation. Using blood clearance and liver accumulation of labeled sheep RBC as a measure of reticuloendothelial activity, we found no evidence that a stimulated reticuloendothelial system was responsible for radioprotection by PhRBC. Some evidence suggested that repair of hemopoietic stem cells was enhanced in mice injected with PhRBC before X irradiation.

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