Unmitigated oxidative stress is deleterious, as epitomized by CCl4 intoxication. In this well-characterized model of free radical-initiated damage, liver metabolism of CCl4 to CCl3. causes lipid peroxidation, F-ring isoprostane formation, and pathologic leukocyte activation. The nature of the mediator that couples oxidation to the hepatotoxic inflammatory response is uncharacterized. We found that oxidatively modified phosphatidylcholines were present in the livers of CCl4-exposed rats and not in livers from control animals, that CCl4 metabolism generated lipids that activated 293 cells stably transfected with the human platelet-activating factor (PAF) receptor, and that this PAF-like activity was formed as rapidly as isoprostane-containing phosphatidylcholine (iPC) during oxidation. iPC and the PAF-like activity also had similar chromatographic properties. The potential for iPC activation of the PAF receptor has been unexplored, but we conclude that iPC themselves did not activate the PAF receptor, as phospholipase A1 hydrolysis completely destroyed iPC, but none of the PAF-like bioactivity. Oxidatively fragmented phospholipids are potent agonists of the PAF receptor, but mass spectrometry characterized PAF as the major inflammatory component coeluting with iPC. Oxidatively fragmented phospholipids and iPC are markers of free radical generation in CCl4-intoxicated liver, but PAF generation by activated hepatic cells generated the inflammatory agent.