Cats are important in the life cycle of Toxoplasma gondii because they are the only animal species known to excrete resistant isosporan oocysts. The oocysts are infectious to many mammals, birds and man. Cats shed oocysts in feces for a week following eating toxoplasma infected meat and then become asymptomatic carriers of infection. "Immune" cats shed fewer or no oocysts on re-infections. The major objective of this application is to study immunity to Toxoplasma infection in cats. Cross shedding in feces will be used as a measure of protective immunity in cats.