Seven pigs inoculated endobronchially with Actinobacillus pleuropneumoniae (App) serotype 1 developed acute and subacute necrotizing pleuropneumonia. When treated with high doses of atropine (0.25 mg per kg) and/or xylocaine spray using a bronchoscope, which suppressed mucus secretion and ciliary activity, the pigs showed severe pleuropneumonia and 2 treated pigs died within 36 hr after inoculating 320 colony forming units (CFU)/ 2 ml of App serotype 1. Histopathologically, their lungs had alveolar and interlobular edema and intravascular fibrinous thrombosis. In the surviving pigs, the lymph nodes had App antigens in the germinal centers corresponding closely with activated follicular dendritic cells and increased in the number of IgG- and IgM-containing cells. The bacterial antigens were also observed as small sized granules in the cytoplasm of bronchoalveolar macrophages. These findings suggest that the attachment of App to the mucosal surface may be crucial in the development of pneumonic lesions.