To study competition between naïve and memory T cells, we examined proliferation of adoptively transferred naïve CD8⁺ T cells in lymphopenic recipients or recipients containing a clonal population of CD8⁺ T cells. We find a hierarchy in the extent of T cell proliferation that appears to correlate with the strength of T cell receptor (TCR)-self-peptide-MHC (pepMHC) interactions. CD8⁺ T cells also proliferate in recipients containing a full complement of CD8⁺ cells with a different TCR if the transferred T cells experience stronger TCR-self-pepMHC interactions than the resident T cells. Furthermore, CD8⁺ T cells proliferate in recipients that contain memory CD8⁺ cells with a different TCR, but in this case the relative strengths of TCR-self-pepMHC interactions are not as critical. In contrast, CD8⁺ T cells do not proliferate significantly in recipients harboring naïve or memory CD8⁺ cells that bear the same TCR as the transferred cells. These results suggest that, among naïve T cells and between naïve and memory T cells, CD8⁺ cells having the same TCR compete for both self-pepMHC and cytokines, whereas TCR-different CD8⁺ cells compete for cytokines. These competitive relationships probably help maintain the size and TCR diversity of naïve and memory T cell populations required for optimal immune responses.