A comparative study has been made of the chemical composition of cell walls from two strains ofEscherichia coli—one rough strain which was very sensitive to killing and lysis by normal human serum, and one smooth strain which was relatively insensitive to killing and was not lysed at all. The major components in the cell walls of both strains were protein and (or) polypeptide (70–80%) and lipid (14%), no significant differences in either component being detected between the two strains. A marked difference was, however, detected in the lipopolysaccharide fraction which was present to the extent of only 1% in the walls of the rough, serum-sensitive strain and 9% in the smooth, serum-resistant strain. Moreover, the two lipopolysaccharides were qualitatively different in sugar composition, that from the resistant strain yielding glucose, galactose, rhamnose, and two minor sugar components, while that from the sensitive strain yielded glucose only. These findings are discussed in relation to the three-layer theory for the structure of the coli cell wall and in relation to the serum factors (antibody, properdin, complement, and lysozyme) which may participate in the destruction of bacteria by serum.