Cultured porcine neutrophils were incubated in the presence of eitherActinobacillus pleuropneumoniae hemolysin protein (HP) in cell-free culture supernatant fluid or highly purified lipopolysaccharide (LPS) for times ranging from 2 to 15 min, followed by fixation and embedding for electron microscopy. Compared with untreated controls, neutrophils incubated in the presence of HP showed a progressive degeneration as evidenced by swelling, loss of cytoplasmic extensions, cytoplasmic vacuolation, and clumping of granules. Nuclear changes included the appearance of perinuclear spaces, nuclear membrane discontinuities, and disappearance of heterochromatin. These degenerative changes were cumulative and complete by 8 min of incubation time. In contrast, neutrophils incubated in LPS were unaffected compared with controls. Changes observed in neutrophils exposed to HP were similar to observed progressive changes in alveolar neutrophils from pigs exposed to aerosols ofA. pleuropneumoniae.