A purified antigenic protein of Eimeria necatrix free of Eimeria necatrix sporozoites or sporocysts, which induces in a chicken an immune response conferring protection against infection of Eimeria tenella, the protein having a molecular weight of about 26,000 daltons and being composed of two polypeptides joined by a disulfide bond, one of the polypeptides being characterized by a molecular weight of about 18,000 daltons, by a blocked N-terminal amino acid not accessible to Edman degradation, and by the amino acid sequence set forth in FIG. 5, and the other of the polypeptides being characterized by a molecular weight of about 8,000 daltons and by the amino acid sequence set forth in FIG. 5.