In this study the effect of chicken egg yolk antibody (IgY) against Cryptosporidium parvum infection was examined. IgY sample was prepared from eggs of chickens immunized with C. parvum oocyst antigens. In vitro, antibody-treated sporozoites showed reduced binding to Caco-2 cells and lost vitality. These phenomena were not observed with a control IgY sample prepared from eggs of non-immunized chickens. Scid mice orally administered with the antibody demonstrated partial reduction in oocyst shedding after challenge with 10(3) oocysts. IgY, however, could not eliminate the infection after 17 days of continuous treatment. The potentials of using specific IgY for treatment and prevention of cryptosporidiosis were discussed.