The glycoprotein CD8 is believed to play an important role in the maturation and function of MHC class I‐restricted T lymphocytes. CD8 has been proposed to function as a co‐receptor of the TcR to participate in signal transduction, possibly through its cytoplasmic domain that binds to protein tyrosine kinase p56^lck. A T cell‐specific transgene encoding CD8α truncated at the cytoplasmic domain (“tailless CD8α”), was introduced into CD8α‐deficient mice. This animal model was used to study the role of the CD8 cytoplasmic domain in T cell ontogeny and function. “Tailless CD8α” was expressed on the cell surface of thymocytes and peripheral T cells. A small population of peripheral CD4⁻ T cells (6% of T lymphocytes) was found to have cell surface expression of “tailless CD8α” and endogenous CD8β indicating that these cells may belong to the CD8⁺ T cell lineage. A consistent result was obtained from CD8α‐deficient mice bearing the “tailless CD8α” and the MHC class I‐restricted 2C TcR transgenes. A small population of CD4 T cells expressing CD8β the “tailless CD8α” and the 2C TcR transgenes was present in the periphery of these mice in a selecting background, but was absent in a deleting background. When “tailless CD8α” mice were infected with lymphocytic choriomeningitis virus (LCMV), the peripheral CD8⁺ CD4⁻ T cell subset expanded dramatically and a significant LCMV‐specific cytolytic activity was detected. The results suggest that the cytoplasmic portion of CD8α is not absolutely required but dramatically enhances the eficiency of thymic maturation of CD8⁺ T cells. The lack of CD8α cytoplasmic domain in peripheral CD8⁺ T cells does not abolish the generation of cytotoxicity in response to an in vivo LCMV infection, although the cytolytic activity is slightly reduced compared to that in control mice.