Technical Abstract: Recombinant chicken interferon-gamma (IFN-g) was produced in CHO-K1 or Spodoptera frugiperda (SF9) insect cells by transfection with a pcDNA vector or baculovirus vector (SF9-IFN-g) carrying the chicken IFN-g gene. Biological activity of recombinant chicken IFN-g was shown by its inhibition of vesicular stomatitis virus-induced cytotoxicity of chicken embryonic fibroblast cells in vitro. To investigate the role of chicken IFN-g during Eimeria infection, CHCC-OU2 chicken fibroblast cells either pretreated with chicken IFN-g or stably transfected with the chicken IFN-g gene were infected with E. Tenella sporozoites. IFN-g demonstrated significant reductions in intracellular sporozoite development without affecting sporozoite invasion of host cells. Furthermore, chickens treated with recombinant chicken IFN-g showed decreased cocyst production and significant improvement in body weight gain following Eimeria challenge infection. These results provide the first direct evidence that chicken IFN-g exerts an inhibitory effect against Eimeria and provides a ratiional basis for use of this cytokine as a vaccine adjuvant against coccidiosis.