Actinobacillus pleuropneumoniae is the causal agent of porcine contagious pleuropneumonia (PCP). The infection produces important economic losses in porciculture due to its high morbidity and mortality. Survivors are asymptomatic carriers infectious to other pigs and have low alimentary conversion. The causative agent possesses several virulence factors: adhesion fimbriae, lipopolysaccharide of the outer membrane, capsule, and cytolysins. In addition, our group has reported secretion proteases of a wide pH range of activity. These proteases degrade different substrates such as porcine gelatin, hemoglobin and IgA, and bovine or human hemoglobin. To control PCP dissemination, farmers require serodiagnostic tests which detect carriers and discriminate between vaccinated and infected animals. Bacterines used as immunogens are serotype specific and do not prevent the infection. Genes have been cloned that codify a cohemolysin, cytolysins, and an iron-binding protein. We have cloned A. pleuropneumoniae genes using the expression plasmids pUC19 and Bluescript, in Escherichia coli Q358 and DH5 alpha; the screening for antigen production was made in four groups of pigs (vaccinated, experimentally infected, naturally infected, and from slaughterhouses); two E. coli clones expressed polypeptides recognized by sera from all the groups.