An anti-Giardia lamblia (G. lamblia) antibody in chicken egg yolk (anti-GL IgY) was developed and its in vitro and in vivo effectiveness was investigated. Anti-GL IgY expressed the ability to agglutinate Giardia lamblia trophozoites with significant reduction of their movement. In addition, IgY reduced the attachment capacity of G. lamblia trophozoites to intestinal cells in an in vitro attachment inhibition assay in a dose-dependent manner. The protective efficacy of anti-GL IgY was investigated in experimentally induced infection in mice following oral application of G. lamblia trophozoites. Anti-GL IgY improved the body weight gain of the mice in comparison to the infected group. The output of cysts was reduced significantly. These results indicate that anti-GL IgY has a protective effect against experimentally induced giardiasis in mice. This effect might be due to blocking the binding of G. lamblia to the host cells. We show the first evidences about the effect of a specific antibody developed in chicken egg yolk on the protozoan G. lamblia.