The serum dye test by Sabin-Feldman used in the toxoplasmosis serology was adapted to the Eimeria tenella system. The antibodies detected by this specific test reflect a protective immune response. All animals immunized with virulent and attenuated Eimeria tenella oocyst antigens demonstrated serologically immune reactions. Antigen reactivities over 50% correlate with the grade of protection of animals in challenge test.