The addition of bacterial lipopolysaccharide (LPS) from Salmonella minnesota R595 to rabbit plasma results in a marked reduction of the hydrated buoyant density of the parent R595 LPS, from 1.38 to less than 1.2 g/cm3. Using immunopurified anti-R595 LPS antibody covalently linked to Sepharose 4B, we were able to separate the altered R595 LPS (d less than 1.2 g/cm3) from the remainder of the plasma proteins by elution of the bound material with 2.5 M KSCN. The KSCN eluate was shown to have a d less than 1.2 g/cm3 and to contain both R595 LPS as well as protein and lipid characteristic of high density lipoprotein (HDL). The major protein in the KSCN eluate is a single polypeptide chain with an apparent molecular weight of 26,000 in sodium dodecyl sulfate and an amino acid composition essentially identical to that of apoprotein AI, the major protein of rabbit HDL. The lipid composition of the KSCN eluate is similar to that of HDL, although marked differences in the cholesterol ester/cholesterol ration and the phosphatidyl choline/phosphatidyl ethanolamine ratio were observed when the KSCN eluate and rabbit HDL were compared. The formation of this R595 LPS-protein-lipid complex in plasma accounts for the marked reduction in buoyant density found when LPS is added to plasma.