Lipopolysaccharides (LPSs) from 17 different Bacteroides fragilis strains were extracted in a two-step procedure. The first step was a hot phenol-water extraction of whole bacteria, resulting in a crude aqueous phase, which after lyophilization in a second step was extracted with a phenol-chloroform-light petroleum mixture. The resulting LPSs, which were essentially free from contaminating nucleic acid, proteins, and capsular polysaccharide, were investigated for their qualitative and quantitative sugar and fatty acid composition, immunochemical specificity by enzyme-linked immunosorbent inhibition assays, and particle weight by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Of the 17 strains, 13 had LPSs which all contained L-rhamnose, D-glucose, D-galactose, and D-glucosamine in approximately the same ratio. Three of these LPSs also contained D-galactosamine. The fatty acid composition was also similar in that the same fatty acids, although in slightly varying proportions, were found in all LPSs. The 13 strains also showed the same specificity in inhibition studies by enzyme immunoassay with rabbit anti-LPS antisera and LPS antigen. The LPS particle weights were also very similar, in the range of what is found for LPSs from rough mutant strains of enterobacteria. Our results suggest that most strains of B. fragilis have similar, if not identical, LPSs with relatively short polysaccharide chains.