Porphyromonas gingivalis, an important pathogen in periodontitis, possesses strong hemagglutinating activity. This is mediated by a variety of hemagglutinating molecules on its cell surface. We previously succeeded in a cloning the gene encoding 40-kDa outer membrane protein (40-kDa OMP) from P. gingivalis 381. To clarify the pathological role of 40-kDa OMP, we examined its hemagglutinating activity. The recombinant 40-kDa OMP (r40-kDa OMP) was highly purified and used to raise a rabbit antiserum, which was then purified by r40-kDa OMP affinity chromatography. Although the r40-kDa OMP itself did not show hemagglutinating activity, a cross-linked polymeric form had strong activity. Finally, the affinity-purified anti-r40-kDa OMP antibody inhibited hemagglutination caused by P. gingivalis vesicles. These findings suggest that 40-kDa OMP is a novel hemagglutinin that plays an important role in the pathogenicity of P. gingivalis.