We have previously shown that in an animal model of sepsis positively and negatively charged groups on polysaccharide A of Bacteroides fragilis are essential for the induction of intra-abdominal abscess formation (A. O. Tzianabos, A. B. Onderdonk, B. Rosner, R. L. Cisneros, and D. L. Kasper, Science 262:416-419, 1993). By introducing chemical modifications into the structures of B. fragilis polysaccharide B as well as other abscess-inducing bacterial polysaccharides, we observed the following. (i) The presence of a nonacetylated free amino group on these polysaccharides appears to be required for abscess induction. (ii) No specific type of negatively charged group is essential to abscess induction by these polysaccharides. (iii) The density of free amino groups on these polysaccharides influences this pathobiologic host response.