Objectives of this investigation were (a) to characterize a naturally occurring plant-particle pneumonia in swine and (b) to reproduce the disease in experimental pigs using soybean particle suspensions;The incidence and lung histopathology of natural plant-particle pneumonia were determined by examination of lung tissues from 62 confinement-reared, self-fed pigs; 26 confinement-reared, floor-fed pigs; and 444 pig necropsies accessioned during a 5 year period;Characteristics of experimental soybean-induced plant-particle pneumonia were determined by clinical observations and gross, histological, ultrastructural and microbiological examinations of the respiratory tract. Ninety-three pigs given soybean particles transtracheally and 36 control pigs were studied;In the naturally occurring disease, plant particles were associated with granulomatous lung lesions in 25 of 62 self-fed pigs. Particles morphologically consistent with soybean were found in 7 of the 25 pigs. Six of 9 pigs among 444 pigs examined retrospectively had soybean particles in granulomatous lung lesions. Microscopically, lesions in affected pigs from both groups consisted of neutrophils admixed with macrophages and multinucleated giant cells in alveolar and bronchiolar exudate. Plant particles 20 to 100 (mu)m were free in alveoli, in phagocytes, and in granulomas;Experimentally induced plant-particle pneumonia was characterized clinically by coughing and dyspnea. At necropsy, there was gross bullous emphysema, lung hemorrhage, and yellow to gray lung mottling. Microscopically, there was a sequential transition from acute bronchopneumonia to granulomatous pneumonia. Ultrastructurally, there were interstitial foreign body granulomas which formed when multinucleated giant cells became entrapped by connective tissue after alveolar epithelium was destroyed. When pigs were given multiple doses of soybean, lesions were simultaneously suppurative and granulomatous, similar to those seen in the natural disease.