Two distinct types of T cell antigen receptors have been identified: the alpha/beta heterodimer found on functional helper and cytotoxic T cells, and the gamma/delta heterodimer. The latter is first detected approximately 2 days before the appearance of cell surface alpha/beta heterodimer during T cell ontogeny. In adult thymus it is found mainly in the least mature cells. The gene shows systematic rearrangement in early thymocytes and appears to use V gene segments of the TCR alpha chain family, well before any C(alpha) message of protein is detected. RNA from this locus is expressed at a high level in early thymocytes and adult 'double-negative' cells, but not in mature T cell populations.