Study of the chemical properties of the pyrogenic component of rabbit polymorphonuclear leucocytes reveals it to contain an essential, non-dialyzable protein which: (a) is precipitated by perchloric acid, (b) is removed by extraction with phenol, (c) is soluble in 50 per cent methanol and 33 per cent saturated ammonium sulfate, and (d) is destroyed by the proteolytic action of both trypsin and pepsin.

By combined chemical and chromatographic techniques the leucocytic pyrogen has been purified approximately 50-fold. The partially purified material contains less than 1 per cent carbohydrate, is resistant to periodate oxidation, is unaffected by extraction with butanol and contains at least two immunologically active components when tested by the Ouchterlony gel-diffusion technique. Its chemical properties distinguish it from other known pyrogenic substances which have been implicated in the pathogenesis of fever.

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