Coccidiosis is caused by intra-cellular infection of Eimeria spp., which goes through a complex life cycle in the intestinal mucosa of infected hosts. Specific immunoglobulins (IgY) could be produced in egg yolk by immunizing hens with specific antigens. In the present study, we cloned the E. maxima gam56 gene, expressed the GST-GAM56 fusion protein and raised IgY to GST-GAM56 in hens. The anti-GST-GAM56 IgY antibody was isolated and used to treat chickens infected with E. maxima oocysts. Intramuscular injection of the antibodies provided minimal protection against parasite infection. However, oral dosing of the IgY 3 or 5 d after oocyst inoculation significantly improved body weight gain, reduced oocyst output and intestinal lesion score were reduced at 3 or 5 d after oocyst challenging, compared to the untreated control group. Our findings suggest that the IgY to gam56 could be an effective prophylactic or therapeutic agent against E. maxima infection in chickens and should have a practical application value.