Negative as well as positive co-stimulation appears to play an important role in controlling T cell activation. CTLA-4 has been proposed to negatively regulate T cell responses. CTLA-4-deficient mice develop a lymphoproliferative disorder, initiated by the activation and expansion of CD4+ T cells. To assess the function of CTLA-4 on CD8+ T cells, CTLA-4(-/-) animals were crossed to an MHC class I-restricted 2C TCR transgenic mouse line. We demonstrate that although the primary T cell responses were similar, the CTLA-4-deficient 2C TCR+ CD8+ T cells displayed a greater proliferative response upon secondary stimulation than the 2C TCR+ CD8+ T cells from CTLA-4 wild-type mice. These results suggest that CTLA-4 regulates antigen-specific memory CD8+ T cell responses.