Monoclonal antibodies against purified fimbriae from this organism blocked its adherence to buccal epithelial cells. Three clones of monoclonal antibodies against these fimbriae were selected for use. The isotype of the three was IgG1 kappa chain. The antibodies reacted with fimbriae or their partially dissociated oligomers, but not with their constituent monomers (43 K protein, fimbrilin) or with other B. gingivalis 381 components, in an enzyme-linked immunosorbent assay or by immuno-blotting. The antibodies agglutinated only B. gingivalis 381 cells and no other species of Bacteroides. The purified immunoglobulin G (IgG) antibodies inhibited bacterial adherence to the human buccal epithelial cells, but had no effect on bacterial haemagglutination to various animal and human erythrocytes. The papain-cleaved Fab fragment, which did not allow cell to cell cross-linking, also inhibited adherence of B. gingivalis 381 but did not interfere with haemagglutination. Thus the fimbriae of B. gingivalis 381 may be responsible for adherence to epithelial cells, which supports the notion that a different type of fimbria or a lectin-like protein may be acting as haemagglutinin in this bacterium.