Extraction of the cell envelope of E. coli with 1% sodium dodecyl sulfate yielded a lipopolysaccharide protein that was purified to homogeneity by conventional techniques. Analysis of the pure protein indicated that it is a complex lipopolysaccharide protein with the following molar ratios of constitutents: 3-deoxyoctulosonate, 1.0; glucosamine, 1.3; neutral sugar (glucose + heptose), 1.0; organic phosphate, 2.3; and amino acid, 21. On a quantitative basis, all of the 3-deoxyoctulosonate present in the cell envelope preparation was solubilized in hot sodium dodecyl sulfate and subsequently accounted for in its entirely in the isolated protein component. After incubation of the cell-wall particulate fraction of cells grown on [(3)H]glucosamine with UDP-[(14)C]galactose under conditions designed to measure galactosyl transferase activity, the isolated lipopolysaccharide protein contained all of the [(14)C]galactose that was incorporated during the incubation. We concluded that the lipopolysaccharide of this organism occurs in the outer cellenvelope membrane exclusively in the form of lipopolysaccharide protein.