The "endogenous serum pyrogen" that appears in the circulating blood after a single intravenous injection of endotoxin does not produce leukopenia in normal animals, fails to provoke the local Shwartzman reaction, and elicits no "tolerance" when injected daily. Suppression of the febrile response to endotoxin by prednisone does not prevent the appearance of pyrogen in the blood.

Animals given large amounts of endotoxin daily continue to respond with high fevers despite failure of endogenous serum pyrogen to appear in detectable amounts after the first two or three injections. Analysis of the response to daily injections shows clearly that the fever during the first 2 hours after administration of endotoxin is unrelated to levels of endogenous serum pyrogen; in contrast, the magnitude of the fever after the 2nd hour correlates well with endogenous pyrogen in some instances. The leukopenic response to endotoxin could not be correlated with the appearance of endogenous serum pyrogen.

The differences between endotoxin and endogenous pyrogen and the similarities between leukocyte extracts (sterile exudates) and endogenous pyrogen are summarized and discussed. Dissociation of the febrile response to bacterial endotoxin and levels of endogenous serum pyrogen are discussed and it is concluded that a mechanism involving both direct and indirect action of endotoxins offers the best explanation for the pyrogenic action of these bacterial products.

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