The pathogenic potentials of encapsulated and unencapsulated strains of Bacteroides fragilis were compared by use of a rat model of intraabdominal sepsis. Implantation of encapsulated B. fragilis alone resulted in abscesses in most recipients, whereas unencapsulated strains seldom produced this effect unless they were combined with another organism. Implants of heat-killed, encapsulated B. fragilis also resulted in abscess formation. Subsequent experiments suggested that the abscess-potentiating ability of encapsulated B. fragilis is related to the capsular polysaccharide. Implantation of 200 microgram of the purified capsular material alone or in conjuction with unencapsulated strains caused abscess formation in a majority of animals. Comparable results were not obtained with capsular polysaccharide from Escherichia coli O7:K1(L)"NM or with heat-killed Streptococcus pneumoniae type III. The capsular polysaccharide of B. fragilis appears to potentiate abscess formation and may represent a virulence factor for this species.