Description: The aim of this study was to quantify and compare interferon-? (IFN-?) concentrations in the serum of clinically normal cats infected with feline coronavirus (FCoV) with its concentration in the sera and effusions of cats with feline infectious peritonitis (FIP), a disease associated with infection with a mutated form of FCoV.. - Clinically normal FCoV-infected cats living in catteries with a high prevalence of FIP had the highest serum IFN-? concentrations. The serum concentration of IFN-? was not significantly different in cats with FIP compared with clinically normal FCoV-infected animals living in catteries with a low prevalence of the disease. Moreover, the concentration of IFN-? was significantly higher in the effusions than in the serum of cats with FIP, probably due to IFN-? production within lesions. These findings support the hypothesis that there is a strong, СsystemicТ cell mediated immune response in clinically normal, FCoV-infected cats and that a similar process, albeit at a tissue level, is involved in the pathogenesis of FIP