Acute phase proteins (APPs) and immune responses in pigs after experimental infection with H1N2 swine influenza virus (SwH1N2) were studied. Eight piglets were infected intranasally with SwH1N2. Four served as controls. Antibodies against swine influenza virus (SIV)s were measured by hemagglutination inhibition assay. The proliferation assay was used to measure influenza-specific cell-mediated response. Hematological parameters were measured on a hematology analyzer. C-reactive protein (CRP), haptoglobin (Hp), serum amyloid A (SAA) and pig major APP (Pig-MAP) concentrations in serum were measured using commercial ELISAs. Antibodies against SwH1N2 in the serum of infected pigs were detected from 7 dpi. SwH1N2-specific T-cell response was observed from 5 dpi. A significant drop in lymphocyte numbers and an increase in medium-sized cell (MID) counts with no accompanying leukopenia was observed in all infected pigs from 3 to 7 dpi. In infected pigs, concentrations of CRP, Hp and SAA increased significantly when the greatest amounts of virus were shed (from 1 to 3 dpi). The level of Pig-MAP remained unchanged during study. The significant positive correlation found between maximum concentrations of SAA in serum and lung scores, makes SAA a potentially useful indicator in experimental infection studies (e.g. vaccine efficiency investigations) or as a marker for disease severity, but to confirm this hypothesis more studies are needed.