Cytoenzymatic changes in phagocytes present in organs and tissues with signs of inflammation have been presented, as well as the ultrastructural picture of peritoneum in cats affected with feline infectious peritonitis. A significant increase of acid phosphatase, lysozyme, beta-glucuronidase and nonspecific alpha esterase in lysosomes of histiocytes, macrophages and neutrophils were discovered. Ultrastructural examinations revealed an increase of collagen fibres under the epithelium and a subsequent decrease of integrity and the detachment of mesenthelial cells that made the peritoneum less resistant to infection.