The development of a technique for producing a pure Bacteroides fragilis infection in mice is described. The infection produces large subcutaneous abscesses at the site of bacterial injection in approximately 90% of the mice. The abscess can be seen as an obvious swelling within 5-7 days post-injection. The infection was initiated by injection of pure cultures grown in a semisolid medium. Similar infections were also produced with pure cultures of B. distasonis, B. ovatus, B. thetaiotaomicron, and B. vulgatus.