Trauma, thermal injury, and nonlethal doses of endotoxin can promote the translocation of endotoxin across the mucosal barrier of the colon into the mesenteric lymphatics and systemic circulation. Bacterial endotoxemia induces changes in lipid metabolism, including an increase in circulating triglyceride-rich lipoproteins. Because cholesterol-rich lipoproteins can neutralize the toxic activity of endotoxin, both in vitro and in vivo, we asked whether triglyceride-rich chylomicrons can inhibit endotoxin activity in vitro as measured by a chromogenic Limulus assay. We tested the effect of intact versus heat-denatured chylomicrons on the in vitro activity of increasing concentrations of Escherichia coli (055:B5) endotoxin. Intact chylomicrons inhibited up to 12-fold the detection of as much as 1 microgram of endotoxin/mg of chylomicron triglyceride, compared to denatured chylomicrons (P less than 0.001). This study shows that chylomicrons are potent inhibitors of endotoxin activity in vitro. Because translocated endotoxin from the colon associates with gut-derived chylomicrons in the mesenteric lymphatics, this may represent a natural defensive mechanism against endotoxemia of enteric origin.