A high-molecular-weight (approximately 150,000) protein was selectively isolated by acid extraction from the cell surface of Wolinella recta and purified by negative adsorption on DEAE-cellulose and gel filtration. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis showed that this protein was found in W. recta but not in other Wolinella species, such as W. curva and W. succinogenes. Sera from patients with periodontitis reacted strongly with this protein antigen, whereas sera from healthy donors showed little or no reactivity, as determined by immunoblotting analysis. In serum, titers of immunoglobulin G antibodies to the protein antigen were significantly higher in patients with periodontitis than in periodontally healthy donors, as detected by an enzyme-linked immunosorbent assay.