Lipopolysaccharides (LPS) have been extracted and purified from two anaerobic gram-negative bacteria: Bacteroides oralis and tfusobacterium nucleatum. Chemical analysis of the preparations showed a great proportion of neutral sugars, mainly glucose, in LPS of B. oralis. In rabbits, LPS of B. oralis induced both immunoglobulin M and G antibodies in contrast to LPS of F. nucleatum, to which only immunoglobulin M antibodies were produced. An immunohistochemical method with horseradish peroxidase-labeled antibodies was used to localize LPS antigens at the ultrastructural level. An electron-dense reaction product, representing an immune complex consisting of bacterial surface antigens and specific rabbit immunoglobulin labeled with peroxidase, was surrounding the cell wall, whereas appropriate controls were negative. The findings of the present study show that LPS of Bacteroides are probably bound to a complex, including glucans, in the outer membrane of the cell wall. LPS of Fusobacterium resemble LPS of other gram-negative bacteria.