The major protein present in the isolated outer membrane of Treponema pectinovorum ATCC 33768, MompA, was identified, purified, and characterized. Immuno-gold electron microscopy, using anti-MompA serum, and cell fractionation experiments confirmed the localization of MompA to the outer membrane. MompA was determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis to have a molecular mass of 42 kDa when heat denatured, whereas native MompA formed a number of detergent-stable forms with molecular masses of 71, 76, and 83 kDa. A temperature of 60 degrees C was required to convert the native protein to the 42-kDa form. A number of detergents and chemical agents that are capable of breaking ionic and hydrogen bonds of proteins did not convert native MompA to the 42-kDa species. The native forms of the protein were resistant to the combined action of proteinase K, trypsin, and chymotrypsin, whereas the 42-kDa form of MompA was not. The N-terminal amino acid sequence of MompA was determined to be DVTVNINSRVRPVLYTT, and database searches did not identify any homology with known protein sequences. Amino acid compositional analysis showed the protein to be rich in proline and glycine, with these amino acids accounting for 28 and 13%, respectively, of the total amino acids. Antiserum raised against the major outer membrane protein of T. denticola GM-1 and ATCC 35405 did not cross-react with MompA, and antiserum raised against MompA did not react with any cellular components of Treponema denticola, Treponema vincentii, or Treponema socranskii. A major outer membrane protein similar in molecular mass to MompA was identified in eight clinical isolates of T. pectinovorum. The major outer membrane protein produced by four of the clinical isolates reacted strongly, by Western blotting, with anti-MompA serum, whereas proteins of the other strains did not.