This paper describes a suppressor T cell factor which protects mice against intraabdominal abscesses caused by Bacteroides fragilis. This soluble cell-free factor (ITF) is derived from splenic T cells from mice immunized with capsular polysaccharide (CP) of B. fragilis. Mice receiving ITF are protected from developing abscesses caused by B. fragilis to the same degree as animals receiving intact immune splenic T cells. The factor appears to be small in molecular size as protective activity is dialyzable through a 12,000-mol wt exclusion dialysis membrane and is present in fractions intermediate between the bed and void volumes of a P2 Biogel column. The protective effect of ITF is antigen-specific to B. fragilis alone. Mice given a complex inoculum of B. fragilis, enterococcus, and another anaerobe develop abscesses even after receiving column-purified ITF. The activity of ITF also is eliminated after adsorption with B. fragilis CP coupled to sheep erythrocytes but not with an unrelated CP coupled to sheep erythrocytes. ITF, therefore, appears to have a binding site for B. fragilis CP. ITF is heat-labile and loses efficacy after protease digestion, suggesting that the active material is a protein. These studies define a suppressor cell factor with activity in a model system resembling human disease and offer promise for increased understanding of the diversity of cell-mediated immune systems.