The endotoxic activity of lipopolysaccharide (LPS) extracted from the envelope of Bacteroides fragilis is low compared with that of LPS from Escherichia coli, Salmonella, and other Enterobacteriaceae. Thus, pyrogenicity, the ability to prepare for or provoke the local Shwartzman reaction, and the ability to induce the production of interleukin 1 are reduced by 100- to 1,000-fold. Structural analyses of characterized B. fragilis LPS have shown that its lipid A is composed of a beta 1,6-linked D-glucosamine disaccharide that has the following properties: (1) a phosphate group on C1 of the reducing amino sugar, (2) amide- and ester-linked 3-hydroxylated branched and nonbranched long-chain (C15-C17) fatty acids, (3) an average of five fatty acids per glucosamine disaccharide, and (4) the core and O-antigenic saccharide chain linked to C6 of the nonreducing glucosamine residue. Although structurally similar to lipid A of E. coli, the lipid A of B. fragilis differs by its lack of the phosphate group on C4 of the nonreducing amino sugar and by the presence of fewer and different fatty acids. These differences explain the low endotoxic activity of B. fragilis LPS. The core and O-antigenic chain are linked to lipid A via a phosphorylated 2-keto-3-deoxyoctonate (KDO) residue. The saccharide chain is short and is composed of L-rhamnose, D-glucose, and D-galactose, with the O-antigenic specificity determined by a beta 1,6-linked D-galactose oligomer. This O-antigenic specificity was present in 14 of 17 strains of B. fragilis that were investigated.