Lipopolysaccharides (LPS) of the same chemotype isolated from seven strains of Fusobacterium nucleatum were examined by indirect haemagglutination and inhibition of haemagglutination in unabsorbed and absorbed antisera. Four common major antigenic specificities were detected, and two of the LPS examined contained antigenic specificities not shared with any of the other LPS. In addition, a few weak cross-reactions were observed, also with some LPS of other chemotypes.