Actinobacillus pleuropneumoniae is a pathogenic bacterium responsible for a highly contagious and often fatal form of bronchopneumonia in swine. Survival from a natural infection generally results in immunity from further infection by all 12 common serotypes, suggesting the presence of common protective antigens. We have identified one of the antigenic membrane proteins from A. pleuropneumoniae serotype 5, and cloned the gene which encodes it. This gene is found in all 12 serotypes, and encodes a protein with a predicted molecular mass of 30 kDa. Sequence analysis revealed that this antigen has a typical signal sequence characteristic of lipoproteins, and is likely to be secreted and inserted into the periplasmic side of the inner membrane. The gene shows high homology to the surface antigen CjaA of Campylobacter jejuni and to solute binding proteins of the ABC transporter family. The probable role of this protein in substrate binding and transport was supported by the presence of an upstream gene with significant homology to ATP binding proteins of the same family. In Escherichia coli, the cloned gene produced a protein which reacted strongly with convalescent sera from swine infected with A. pleuropneumoniae serotype 5, and weakly with sera from swine infected with serotype 1A or from swine vaccinated with a killed bacterin of serotype 1A or 5. It thus appears that this antigen displays some crossreactivity between serotypes, and may be less exposed in bacterins than in live cells. This protein, designated ApaA, may have an important role in nutrient acquisition and in the pathogenesis of infections caused by A. pleuropneumoniae.