The ability of various leukotrienes, platelet-activating factor and N-formyl-methyl-leucyl-phenylalanine to augment colonic damage induced by 30% ethanol was investigated in the rat. Each of the mediators was tested at a dose of 2 nmol, administered intracolonically in the ethanol vehicle. When colonic damage was assessed 72 hr later, only leukotriene B4 significantly augmented damage compared to the controls. The incidence of ulcers increased from 35% in the control group to 90% in the group receiving leukotriene B4. Leukotriene B4 administration also resulted in significant increases in colonic myeloperoxidase activity and colonic leukotriene B4 synthesis. To assess the possible contribution of infiltrating neutrophils to the increase in colonic leukotriene B4 synthesis that accompanies colonic inflammation, colitis was induced in normal and neutropenic rats by intracolonic administration of trinitrobenzene sulfonic acid. Neutropenia was achieved by treatment with an antineutrophil serum. In the neutropenic animals killed 4 hr after induction of colitis significant changes in leukotriene B4 synthesis were not observed, whereas a fourfold increase was observed in the controls. From these studies we conclude the following: (1) leukotriene B4, at a dose of 2 nmol, can significantly potentiate the colonic ulceration induced by 30% ethanol; (2) this action of leukotriene B4 is not shared by the same dose of the other inflammatory mediators tested; and (3) infiltrating neutrophils are the major source of colonic leukotriene B4 synthesis in a rat model of colitis.