Actinobacillus pleuropneumoniae RTX toxin (Apx) production by A. pleuropneumoniae biotype 2 (NAD-independent) serotype 2 strains was studied. Western blot analysis of culture supernatants of all biotype 2 strains tested revealed the presence of a 103 kDa protein which reacted with a monoclonal antibody against ApxIIA. This protein was also recognized by sera of pigs infected with a biotype 2-serotype 2 strain. Furthermore, antibodies that could neutralize ApxIIA were present in these sera. Proteins corresponding to ApxIA or ApxIIIA were not detected. The effects of a biotype 1-serotype 2 and a biotype 2-serotype 2 strain and their metabolites on the oxidative activity of porcine pulmonary alveolar macrophages (PAM) and polymorphonuclear cells (PMN) were compared using a chemiluminescence (CL) technique. Viable bacteria of both biotypes stimulated the production of oxygen radicals by phagocytes. CL responses were higher for the biotype 1 than for the biotype 2 strain. After having reached a peak value, the oxidative activity decreased until a total inhibition was achieved. Inactivated washed bacteria had no influence on the oxidative activity of phagocytes. In contrast, heat labile factors in culture supernatants of both biotypes stimulated and inhibited the oxidative activity of PAM in a dose-dependent manner. Dilutions of supernatant up to 1/32 of the biotype 2 strain and up to 1/512 of the biotype 1 strain were toxic for PAM, while dilutions from 1/64 to 1/128 of the biotype 2 strain and from 1/1024 to 1/4096 of the biotype 1 strain stimulated the oxidative activity.(ABSTRACT TRUNCATED AT 250 WORDS)