T-cell antigen receptors are associated on T cell surfaces with a complex of proteins called CD3 (formerly T3). Human CD3 consists of at least four proteins, gamma, delta, epsilon and zeta, and all but the latter have been cloned as cDNA. Using standard cloning techniques, together with field inversion gel electrophoresis, we have demonstrated the physical linkage of three CD3 genes. The genes for CD3 gamma and CD3 delta are situated close together, about 1.6 kb apart, organized in a head-to-head orientation. The gene encoding CD3 gamma has been sequenced, and is split into seven exons spread over 9 kb of DNA. Like CD3 delta, CD3 gamma gene has an unusual promoter which lacks a TATA-box and potential Sp1 binding sites. The CD3 gamma-CD3 delta gene pair is within 300 kb of the CD3 epsilon gene, and therefore these genes form a tightly linked cluster in chromosome 11 band q23. The clustering of the CD3 genes may be significant in terms of their simultaneous activation during T-cell development.