The lipopolysaccharide (LPS) of Fusobacterium nucleatumJCM 8532 was isolated by hot‐phenol water extraction. Most of the LPS was extracted in the phenolic phase and shown to be the smooth‐type, whereas the aqueous phase contained mainly rough‐type LPS. The chemical composition of the LPS was similar to that reported in other studies, but D‐quinovosamine, which may be a major component of O‐antigenic polysaccharide, and 3‐deoxy‐D‐manno‐2‐octulosonic acid (Kdo) were detected for the first time by gas chromatography‐mass spectrometry. The biological activities of smooth‐type LPS, including limulus activity, lethal toxicity, pyrogenicity, and B lymphocyte mitogenicity, were comparable to those of enterobacterial LPS. Smooth‐type LPS inhibited the cell growth and DNA synthesis of adult and fetal human gingival fibroblasts in a dose‐dependent manner, suggesting that LPS may play a role in the occurrence of human gingivitis.