Study in vitro of the interaction of bacterial endotoxin with rabbit polymorphonuclear leucocytes has resulted in the following findings:

1. Incubation of endotoxin and leucocytes in saline results in: (a) the release of leucocytic pyrogen, and (b) the inactivation of endotoxin.

2. Cell-free extracts of leucocytes also inactivate endotoxin.

3. Incubation of leucocytes in "physiological" saline causes rapid discharge of leucocytic pyrogen. In contrast, relatively little pyrogen is released by leucocytes incubated in fresh serum.

4. The release of leucocytic pyrogen in serum is markedly stimulated by the presence of endotoxin.

5. Leucocytes obtained from tolerant rabbits interact with endotoxin in essentially the same manner as leucocytes from normal rabbits.

The pertinence of these findings to the pathogenesis of fever and to related information concerning human leucocytes has been discussed.

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