Since endotoxins are potent stimulators of the hematopoietic system, and hematopoietic recovery after irradiation appears to be accelerated in endotoxin-treated mice, the mechanism of endotoxin protection may be closely related to effects on the hematopoietic system. The present studies were conducted to evaluate the effect of endotoxin on the return of leukocyte counts toward normal in irradiated mice and dogs. Typhoid-paratyphoid vaccine was used as the source of endotoxin, and animals were injected 24 hours before X-irradiation. The data show that the endotoxin-treated animals had higher numbers of circulatory granulocytes than did controls between approximately 7 and 17 to 21 days after irradiation. The pattern of the return of granulocyte counts toward normal in endotoxin-treated animals was characterized by a large rise during the second week which coincided in time with the "abortive rise" observed in nontreated animals. The granulocyte counts in endotoxin-treated animals declined after the period of the abortive rise but remained higher than in the nontreated controls and returned to normal earlier than did the counts of the controls.

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