Lipopolysaccharide (LPS) has been isolated from the supernatant fluid after ultracentrifugation of the water phase following phenol‐water extraction ofBacteroides melaninogenicus, strain B10. The purification procedure included digestion of the supernatant fluid with ribonuclease and deoxyribonuclease, gel filtration on Bio‐Gel A‐1.5 m and ion exchange chromatography on DEAE‐cellulose. The prepared LPS did not contain heptose or 2‐keto‐3‐deoxyoctulosonic acid. The sugar components identified were galactose, glucose, mannose, rhamnose, traces of fucose, galactosamine and glucosamine. Neutral sugar and lipid constituted the main parts of the LPS. The isolated LPS sensitized sheep erythrocytes to agglutination in homologous antiserum. Two agar precipitation lines were observed, one of which may correspond to an