Experimental infections were induced out to examine whether Aujeszky's disease virus (ADV) infection in pigs results in a severe pneumonia by Actinobacillus pleuropneumoniae. Intranasal inoculation of ADV (10(6.9) median tissue culture infective dose/head) in 4-month-old primary specific-pathogen-free pigs was followed by the inoculation of A. pleuropneumoniae type 1 (10(3.1) or 10(5.1) colony-forming-units/head). The pigs inoculated with ADV alone developed clinical signs of Aujeszky's disease but not pneumonia, and those inoculated with A. pleuropneumoniae (10(3.1) CFU/head) alone did not develop clinical symptoms and lung lesions. Whereas all the pigs inoculated dually with ADV and A. pleuropneumoniae (10(3.1) CFU/head) showed severe or very severe clinical symptoms and moderate or severe pneumonia and one of them died. The pigs inoculated with A. pleuropneumoniae (10(5.1) CFU/head) alone had severe clinical symptoms and one of the 2 pigs died acutely. Furthermore, all of the 3 pigs inoculated with ADV and A. pleuropneumoniae (10(5.1) CFU/head) showed clinical symptoms and moderate or severe pneumonic lesions and one pig died of disease. It was concluded that the clinical symptoms of A. pleuropneumoniae became severer by concomitant infection with ADV.