Evidence is presented that the mitogen concanavalin A stimulates human lymphocytes to produce a nonpyrogenic lymphokine (LK) that is capable of activating human monocytes but not granulocytes to produce endogenous pyrogen (EP) in vitro. The potency of this preparation should facilitate further studies to purify and characterize this agent and determine its relation to other know LK. It seems likely that this factor, which we have called EP-activating factor (EPAF), plays a significant role in the development of fever in states of delayed hypersensitivity in man.

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