Inflammation is a local immune response to 'foreign' molecules, infection and injury. Leukotriene B₄, a potent chemotactic agent that initiates, coordinates, sustains and amplifies the inflammatory response, is shown to be an activating ligand for the transcription factor PPARα. Because PPARα regulates the oxidative degradation of fatty acids and their derivatives, like this lipid mediator, a feedback mechanism is proposed that controls the duration of an inflammatory response and the clearance of leukotriene B₄ in the liver. Thus PPARα offers a new route to the development of anti- or pro-inflammatory reagents.