The structure of a chicken CD3 chain has been determined by isolating a cDNA clone (T11.15) that encodes a 175-amino-acid-long protein, including the NH2-terminal signal peptide. In Northern blot experiments, the earliest expression of the T11.15 transcript was detected in the thymus at embryonic day 10 (i.e., 1 day after cytoplasmic expression of a CD3 epitope recognized by a specific monoclonal antibody [CT3; Chen, C.L.H., Ager, L.L., Gartland, G.L. & Cooper, M.D. (1986) J. Exp. Med. 164, 375-380], but 2 days before the appearance of clonotypic components of the T-cell antigen receptor). Sequence similarity of this chicken protein sequence compared with that of the known mammalian CD3 gamma and delta polypeptides was 36-39% and 39-40%, respectively. Amino acid sequence alignments between avian and mammalian CD3 revealed maximum conservation in the transmembrane and cytoplasmic domains as well as in the regions flanking the cysteine residues in the extracellular domain, underlining their functional importance. The difficulty of unambiguously assigning this chain to a single mammalian CD3 subunit on the basis of sequence comparison raises the possibility that this polypeptide represents a derivative from an ancestral form of the gamma and delta chains. It is thus possible that a single chain may play the role of both CD3 gamma and delta subunits in the chicken CD3 complex or, alternatively, that gene duplications occurred independently in the avian and mammalian lineages.