The polymorphic mononuclear cells, arranged in whorling or palisading pattern, were usually found in the lung of Actinobacillus pleuropneumoniae-infected pigs. In order to understand the origin and characteristics of these cells, specific-pathogen free pigs were intratracheally inoculated with Actinobacillus pleuropneumoniae at a concentration of 5 x 10(6) CFU, then, sacrificed at 6, 12, 24 and 48 hours later. The cells in the alveolar spaces were observed with light and electron microscope, and cytochemically analyzed for acid phosphatase, alkaline phosphatase, alpha-naphthyl acetate esterase, and Sudan black B stains respectively. The results revealed that a lot of neutrophils were observed in the alveolar spaces at the early stage after inoculation. Twenty four hours later, polymorphic mononuclear cells abundantly appeared. Enzyme cytochemical findings indicated that some of the polymorphic mononuclear cells were macrophages, in which, acid phosphatase and alpha-naphthyl acetate esterase were detected, and others were type II pneumocytes which were positively stained with alkaline phosphatase and Sudan black B. Ultrastructural observation found that many lysosomes appeared in the macrophages' cytoplasm, and type II pneumocyte contained many lamellar bodies. Conclusively, it could be suggested that the polymorphic mononuclear cells were derived from macrophages and type II pneumocytes by cytochemical and electron microscopic examinations.