Interleukin-16 is an inflammatory cytokine synthesized as a precursor protein (pro-IL-16). Based on sequence data from an EST cDNA library prepared from intestinal intraepithelial lymphocytes of Eimeria-infected chickens, we identified a cDNA that contained a full-length open reading frame of pro-IL-16. The encoded protein, predicted to consist of 607 amino acids, showed 86% sequence identity to duck pro-IL-16 and 49-52% identity to various mammalian homologues. By Northern blot analysis, IL-16 transcripts were identified in chicken lymphoid tissues but none of the non-lymphoid tissues examined. A recombinant protein containing the 149 C-terminal amino acids of pro-IL-16, expressed in COS-7 cells, showed chemoattractant activity for splenic lymphocytes.