Intravenous administration of bacterial endotoxins in dogs is followed within 2 hours by the appearance of a fever-producing substance in the blood. This endogenous pyrogen differs from the endotoxins originally administered by its ability to produce fever in tolerant recipients and failure to promote tolerance after repeated daily injections. Endogenous serum pyrogen is destroyed by heating at 90°C. for 30 minutes, and is also inactivated to some degree by incubation at 37°C. for 24 hours. Suppression of fever by aminopyrine does not affect appearance of the endogenous factor. Animals made febrile with dinitrophenol, kaolin, or lysergic acid do not elaborate a fever-promoting substance in the blood. Sterile abscesses, accompanied by elevations in body temperature of the host, are unassociated with detectable amounts of secondary pyrogen in the serum. The absence of endogenous pyrogen in the blood of febrile dogs made leukopenic with nitrogen mustard favors the idea that polymorphonuclear leukocytes injured by endotoxins release the endogenous factor. On the other hand, the finding that the granulocytopenic animals are febrile when no circulating endogenous pyrogen is present, casts doubt upon the essential role of this substance in endotoxin fever.

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