Actinobacillus pleuropneumoniae is the etiological agent of porcine pleuropneumonia, which causes significant losses in industrialized swine production worldwide. Bacterial diagnosis of contagious porcine pleuropneumonia is generally done by bacteriological isolation and cultivation of A. pleuropneumoniae, followed by serological typing which differentiates 15 serotypes (1–3). There are significant differences in virulence among the 15 serotypes. Serotypes 1,5, and also 9 and 11 are involved in particularly severe outbreaks of disease with high mortality and severe pulmonary lesions. Serotypes 2-4, 6-8, 12, and 15 are generally less virulent, causing moderate mortality but relatively strong lung lesions. Serotype 3 seems to be mostly of low epidemiological importance, while the remaining serotypes are isolated only very rarely. The degree of virulence seems to be mainly due to the presence of one or two of the pore-forming RTX toxins ApxI, ApxII, and ApxIII, found in A. pleuropneumoniae, which characterize the different serotypes of A. pleuropneumoniae (4,5) (see Table 1). Toxin ApxI is encoded by the genes apxICABD, with gene A specifying the structural protein toxin, C coding for its activator, and B and D coding for the corresponding type I secretion pathway. ApxI is produced and secreted by the highly pathogenic serotypes 1, 5, 9, and 11, and also in serotype 10 and 14 of A. pleuropneumoniae. ApxII, encoded by apxIICA, is produced by all serotypes except 10 and 14.