Feline infectious peritonitis (FIP) is a devastating, fatal disease of cats that responds poorly to treatment. It is believed that the disease occurs when there is a mutation of the common intestinal coronavirus. There is a depression of the cell-mediated immune response to the mutated virus, which results in an overproduction of antibodies and a loss of the cell-mediated immunity needed to destroy coronavirus-infected macrophages. The polyprenyl immunostimulant (PI) stimulates cell- mediated immunity. Since the publication of an article on the response of three cats with FIP to PI, we have received requests for the PI from owners and veterinarians from throughout the United States. The investigators will treat 20 cats diagnosed with FIP with PI to validate the responses previously seen.