Small CD4⁺8⁺TCR^low thymocytes contain precursors of mature T cells

To evaluate directly the developmental potential of cortical CD4⁺8⁺ thymocytes, highly purified populations of small, nondividing CD4⁺8⁺TCR^low and large, dividing CD4⁺8⁺TCR^high thymocytes from H‐2^d mice expressing a transgenic T cell receptor restricted by H‐2D^b (major histocompatibility complex class I) molecules were transferred into the thymus of normal, nonirradiated H‐2^b recipient mice. The results show that both populations generate CD4⁻8⁺ thymocytes under these conditions, thus providing conclusive evidence that small cortical thymocytes do not represent a “dead end” but an important intermediate stage in T cell development.