The target determinant of a monoclonal antibody (MoAb) to Bacteroides fragilis lipopolysaccharide (LPS) was characterized by inhibition enzyme immunoassay (EIA), immunoblotting (IB), immunofluorescence technique (IF) and electron immunocytochemical (EIC) technique. The MoAb has been shown to react positively with 96% of B. fragilis isolates. LPS preparations from 14 different B. fragilis strains were tested by EIA and IB. Two LPS preparations did not react in any of the tests. In both preparations the D-galactose was either lacking or present in low amount compared with the other LPSs. In addition, inhibition experiments with synthetic disaccharides confirmed that the target determinant is composed of beta-1,6-linked galactose disaccharide. EIC showed that the target of the LPS-MoAb is located on the surface of the outer membrane. These results show that the galactose chain present in LPS isolated from most B. fragilis strains contains the immunodominant antigenic determinant.