The serological and chemical properties of antigens extracted from strains of Bacteroides fragilis and related species belonging to several different DNA homology groups were investigated. Antisera prepared against formalin-treated whole cells suspensions of representative strains were tested against cell suspensions, cell wall preparations, and extracts of homologous and heterologous strains using agglutination, immunodiffusion, and hemagglutination techniques. Serological results indicated that the species were antigenically distinct, although minor cross reactions were observed across species boundaries. However, the serological properties did not appear to distinguish genetic heterogeneity at levels down to approximately 65% homology. Homology groups, including the two B. fragilis subgroups, were relatively homogeneous, although the presence of serotypes within each homology group was suggested. Immunodiffusion tests demonstrated, however, that the "homogeneity" was not always represented by a single "common" antigen but rather implied a mosaic antigen composition for each strain. A minimum of ten and six different antigenic factors were demonstrated on B. fragilis 2393 and 2553, respectively. Similar mosaics were observed with members of the other species. Hemagglutination patterns using crude antigen extracts were also consistent with the antigenic mosaic. Many of the strains were found to be capsulated. Preliminary studies demonstrated a similar sugar composition in the capsular material to that in lipopolysaccharide extracted from the same strain with aqueous phenol. Studies also suggested that the capsule influenced the serological properties of the cell. The chemical make-up of all of the Bacteroides LPS was found to be similar to that of LPS from facultative organisms, although KDO and heptose were not detected in any of the Bacteroides preparations. Electron microscopy of Bacteroides LPS demonstrated a trilaminar structure characteristic of LPS from other gram negative organisms. However, gel-filtration experiments suggested that the Bacteroides LPS may be of a different architecture, particularly regarding the core region. When hydrolyzed by weak acid, Bacteroides LPS behaved differently to LPS from facultative bacteria yielding large amounts of high molecular weight polymers and very little core-type material. B. distasonis and B. thetaiotaomicron displayed lower levels of high molecular weight material, as compared with the other strains, and may represent semi-rough strains. A comparison of the sugar patterns of the antigen extracts from one member of each homology group implied distinct differences among the strains. However, the sugar patterns of two B. fragilis strains which are genetically more related to each other than to the other species, were very similar. The constituents identified in the Bacteroides LPS were glucosamine, galactosamine, glucose, galactose, mannose, fucose, and rhamnose. An unidentified aniline pthalate reacting compound with similar mobility to colitose in chromatography studies was observed in the B. thetaiotaomicron 5482 LPS. It was not thought justified to assign definite chemotypes to individual homology groups due to the possible contamination of LPS preparation with non-LPS material, e.g. capsules or a glycan polymer. / Ph. D.