Technical Abstract: IL-17 is a proinflammatory cytokine produced by activated T cells. A 917 bp cDNA encoding the IL-17 gene was isolated from our EST cDNA library prepared from intestinal intraepithelial lymphocytes (IELs) of Eimeria-infected chicken. It contained a 507 bp open reading frame predicted to encode a protein of 169 amino acids with a molecular mass of 18.9 kDa, a 27 residue NH2-terminal signal peptide, a single potential N-linked glycosylation site, and 6 cysteine residues conserved with mammalian IL-17s. Chicken IL-17 shared 37-46% amino acid sequence identity to the previously described mammalian homologues and also was homologous to the open reading frame 132 of Herpesvirus saimiri (HVS 13). By Northern blot analysis, IL-17 transcripts were identified in a reticulendotheliosis virus-transformed chicken lymphoblast cell line (CU205) and Con A-stimulated splenic lymphocytes but not other chicken cell lines or normal tissues. Conditioned medium from COS-7 cells transfected with chicken IL-17 cDNA induced IL-6 production by chicken embryonic fibroblasts suggesting a functional role for the cytokine in avian immunity.