Attempts were made to study the virulence factors in some strains of B. fragilis group in the rat model. Subcutaneous wound abscesses could be produced by 10(9) CFU/ml of live cells of all the five species of B. fragilis group tested. For determination of virulence factor cellular components (capsular polysaccharide and lipopolysaccharide) of B. fragilis were separated using gel filtration technique and injected in rats. Abscesses could be produced only by capsular polysaccharide fraction suggesting it to be the virulence factor. Studies with transmission electron microscope showed presence of capsular polysaccharide in B. fragilis and B. thetaiotaomicron, it was doubtful in B. distasonis and absent in B. ovatus and B. vulgatus. This suggested that virulence factors other than capsular polysaccharide may be responsible for pyogenic lesions in the noncapsulated species of B. fragilis group. The abscess could not be produced by 10(9) CFU/ml of heat killed cells of non-capsulated B. ovatus and B. vulgatus indicating that in live bacteria, a heat labile factor was responsible for the development of abscess.