Monoclonal antibodies against the various surface antigens of Bacteroides gingivalis were obtained by the fusion of murine myeloma cells (SP2/0-Ag14) with spleen cells of BALB/c mice immunized with the whole cells. Two monoclonal antibodies reacted with lipopolysaccharide, and the other two reacted strongly with capsule antigen. One showed reactivity with the hemagglutinin of the cells. The five monoclonal antibodies reacted with sonicated antigen from all B. gingivalis strains tested. No cross-reactivity of the monoclonal antibodies with antigens from nine species of other black-pigmented Bacteroids strains was observed. An immunoblotting test involving the use of these monoclonal antibodies indicated that the epitope of B. gingivalis lipopolysaccharide was polysaccharide with a high molecular weight of 40,000 to 60,000. The immunoblotting test also demonstrated that the epitopes of capsule antigen and of hemagglutinin were 27,000- and 40,000-molecular-weight proteins, respectively.