The human T-cell antigen receptor-associated T3 complex consists of at least three polypeptides, gamma, delta and epsilon. cDNA clones for the delta-chain have recently been obtained and we have used such clones to isolate the T3 delta gene. The gene has been sequenced and comprises five exons, spread over approximately 3.7 kb of DNA. Transcription of the T3 delta gene is initiated from a non-TATA promoter. S1 mapping experiments and the sequence of a novel cDNA clone show that T3 delta mRNA exists in two forms in T cells. Alternative splicing of pre-mRNA sequences corresponding to the third exon of the T3 delta gene accounts for the two species of mRNA. A putative protein, produced by translation of the shorter mRNA, would lack a transmembrane region and might be secreted or associated with the outer surface of the cell.