Transgenic (TG) mice were generated selectively expressing the gag protein of Friend murine leukemia virus (FMuLV) in the liver. FMuLV gag is also expressed by the FBL leukemia, and is the immunodominant tumor Ag of the CD8+ T cell response in C57BL/6 mice. gag-TG mice expressing FMuLV gag in the liver were tolerant to the protein and failed to generate a CTL response to either FBL or FMuLV gag. This tolerance reflected anergy rather than deletion, as CTL responsiveness could be recovered after four cycles of in vitro stimulation. Adoptively transferred gag-specific T cells were not anergized in gag-TG recipients, as revealed by antitumor activity in vivo. Also, such T cells did not induce detectable autoimmune injury in gag-TG liver cells. These results suggest that the requirements for a tissue Ag to provide a tolerizing stimulus are distinct from those for being the target of a T cell-mediated autoimmune response and that the requirements for induction and maintenance of peripheral tolerance are distinct for naive and primed T cells. That anergic T cells reactive with tumor-associated Ags can be recovered by repetitive in vitro stimulation and can mediate tumor therapy suggests strategies that use such Ags to generate CTL for adoptive immunotherapy should be further developed.