The expression of myeloperoxidase (MPO) was examined in the swine influenza virus (SIV)-infected neutrophils in the lungs of pigs experimentally infected with swine influenza virus (SIV) subtype H1N2 by immunohistochemistry. Five pigs each from the infected and non-infected group were euthanized 1, 3, 5, 7, and 10 days post-inoculation (dpi). Immunohistochemical reactivity was mainly seen in neutrophils. The score for pulmonary histopathological lesions correlated with the score for MPO immunohistochemical reactivity (r ( s ) = 0.962, P < 0.01). In addition, the score for in situ hybridization of SIV nucleic acid correlated with the score for MPO immunohistochemical reactivity (r ( s ) = 0.976, P < 0.01). These results suggest neutrophils are one of the primary effector cells in the early phase of SIV infection in pigs.