Bacteroides fragilis is the anaerobic species most commonly isolated from human clinical specimens, and is resistant to many antimicrobial agents. A monoclonal antibody, mAb4H8 (IgG3), reacting with a specific epitope in the lipopolysaccharide (LPS) isolated from most of the B. fragilis strains, was produced and employed with modified Immuno Polymerase Chain Reaction (mIPCR) for identification of B. fragilis with a detection limit of 10(4) cfu/mL bacterial suspension. A number of bacterial strains were examined, including B. fragilis, Bacteroides spp. other than B. fragilis and other genera. All the B. fragilis strains with the immunodominant (beta1,6-linked D-galactosyl chain) epitope were positive. None of the other strains showed the positive reaction. The results indicate that mIPCR assay with mAb4H8 has a high specificity and high sensitivity.