MHC I-restricted epitopes of chicken ovalbumin (OVA) were originally identified using CD8 T cells as probes. Here， using bioinformatics tools， we identify four additional epitopes in OVA in addition to a cryptic epitope. Each new epitope is presented in vivo， as deduced from the lack of CD8 response to it in OVA-transgenic mice. In addition， CD8 responses to the known and novel epitopes are examined in C57BL/6 mice exposed to the OVA-expressing tumor E.G7 in several ways. No responses to any epitope including SIINFEKL are detected in mice with growing E.G7 or mice immunized with the tumor. Only in E.G7-bearing mice treated with an anti-CTLA4 antibody which depletes tumor-infiltrating regulatory T cells， CD8 responses to SIINFEKL and the novel epitope EKYNLTSVL are detected. Finally， all epitopes fails to treat mice with pre-existing tumors. These observations force an important re-consideration of the common assumptions about the therapeutic value of neoepitopes detected by CD8 responses in tumor-bearing hosts.