We previously showed that the presence of microgram levels of endotoxin inhibited low-density lipoprotein (LDL) uptake and degradation in Hep G2 cells. We also showed that both the polysaccharide and lipid A parts of endotoxins are needed for the inhibitory effects of endotoxins on cellular LDL uptake. The current study was carried out by inclusion of lipoprotein-free plasma (LFP) in tissue culture medium to observe the modulatory influence of non-lipoprotein factor(s) on endotoxin-induced inhibition of endocytotic catabolism of LDL in Hep G2 cells. We found that LFP dramatically promotes the inhibitory effect of endotoxins with a complete polysaccharide, but has no influence on the effect of the Re mutant endotoxin (from S. minnesota Re595), which lacks polysaccharide. By using gel-filtration chromatography, agarose electrophoresis and agarose isoelectric focusing, we further showed that in the presence of LFP, both the endotoxins with a complete polysaccharide and the Re mutant endotoxin complex with and anionize LDL, while in the absence of LFP, these endotoxins poorly interact with LDL. Thus, endotoxin inhibits cellular endocytotic catabolism of LDL by forming LDL-endotoxin complexes, and LFP enhances endotoxin-induced inhibition of endocytotic catabolism of LDL by promoting the interaction between endotoxin and LDL. In addition, our finding that the Re mutant endotoxin also interacts with LDL to form LDL-endotoxin complexes, but has no significant effect on LDL uptake and degradation, further supports the notion that both the polysaccharide and lipid A parts of endotoxins are needed for the inhibitory effects of endotoxins on cellular LDL uptake.