Experimental animal model systems have been used by many investigators to explore the pathogenicity of obligate anaerobes. During the last 15 years, research in our laboratory has utilized an experimental model for intraabdominal sepsis to define the contribution of obligate anaerobes to the infectious process. These studies have shown that obligate anaerobes are important components of the polymicrobic flora present during such infection. Moreover, certain anaerobes, such as Bacteroides fragilis, possess specific virulence factors, such as the capsular polysaccharide, that appear to be important to the infectious process. More recent research has used modifications of the original model system to evaluate the host immune response to B. fragilis. These studies indicate that immunization with the capsular polysaccharide provides a T cell-dependent immunity to abscess development when animals are challenged with B. fragilis. It has also been shown that the killing of B. fragilis is T cell dependent. The observations made with regard to B. fragilis in this animal model system are discussed.