CD4 and CD8 have been useful surface markers for alpha/beta T cell maturation. In an alpha/beta T cell receptor (TCR) transgenic SCID mice system, it has been shown that alpha/beta TCR alone is sufficient to induce CD4 and CD8 surface expression on thymic T cells. Although the late embryonic thymic gamma/delta T cells are predominately single and double positive, it has not been clear if gamma/delta TCR has a similar capacity. In this study, we show that when transgenes encoding the earliest embryonic gamma/delta TCR are coexpressed with the SCID defect, the gamma/delta transgenes promote the appearance of both the CD4-8- and CD4+8+ T cells in the thymus. Furthermore, the expression of CD4 and CD8 does not require continuous surface gamma/delta TCR expression. These results indicate that gamma/delta TCR alone can promote the CD4/8 surface expression, and may suggest a role for gamma/delta T cells in initiating normal thymic ontogeny.