An increase in the number of endogenous hematopoietic spleen colonies present on the spleen of mice 10 days after irradiation is induced by injection of foreign plasma or endotoxin or by bleeding before irradiation. This study was initiated to determine if these factors influence the colony-forming cell compartment in a similar or dissimilar fashion. A single injection of endotoxin was as effective as repeated doses of endotoxin, whereas repeated plasma injections or repeated phlebotomies were more effective than when given as a single stimulus. An injection of endotoxin given 1 hour after irradiation increased the number of endogenous spleen colonies, whereas plasma injection and bleeding had no effect when given at this time. In unirradiated mice, a single injection of plasma had no effect on spleen weight or spleen iron uptake, but bleeding increased iron uptake, while endotoxin increased spleen weight. The total number of transplantable colony-forming cells extractable from a spleen or humerus of unirradiated mice was decreased by endotoxin, unchanged by plasma, and reduced in the humerus but increased in the spleen by bleeding. All three stimuli improved postirradiation survival. However, the radioprotective effect of plasma and bleeding was abolished if mice were splenectomized before the stimulus was given, while splenectomy did not influence the effect of endotoxin.

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