C57BL/6 (H-2^b) mice generate type-specific cytolytic T-lymphocyte (CTL) responses to an immunodominant K^b-restricted epitope, KSPWFTTL located in the membrane-spanning domain of p15TM of AKR/Gross murine leukemia viruses (MuLV). AKR.H-2^b congenic mice, although carrying the responder H-2^b major histocompatibility complex (MHC) haplotype, are low responders or nonresponders for AKR/Gross MuLV-specific CTL, apparently due to the presence of inhibitory AKR.H-2^b cells. Despite their expression of viral antigens and K^b, untreated viable AKR.H-2^b spleen cells cause dramatic inhibition of the C57BL/6 (B6) antiviral CTL response to in vitro stimulation with AKR/Gross MuLV-induced tumor cells. This inhibition is specific (AKR.H-2^b modulator spleen cells do not inhibit allogeneic MHC or minor histocompatibility antigen-specific CTL production), dependent on direct contact of AKR.H-2^b cells in a dose-dependent manner with the responder cell population, and not due to soluble factors. Here, the mechanism of inhibition of the antiviral CTL response is shown to depend on Fas/Fas-ligand interactions, implying an apoptotic effect on B6 responder cells. Although B6.gld (FasL) responders were as sensitive to inhibition by AKR.H-2^b modulator cells as were B6 responders, B6.lpr (Fas) responders were largely insensitive to inhibition, indicating that the responder cells needed to express Fas. A Fas-Ig fusion protein, when added to the in vitro CTL stimulation cultures, relieved the inhibition caused by the AKR.H-2^b cells if the primed responders were from either B6 or B6.gld mice, indicating that the inhibitory AKR.H-2^b cells express FasL. Because of the antigen specificity of the inhibition, these results collectively implicate a FasL/Fas interaction mechanism: viral antigen-positive AKR.H-2^b cells expressing FasL inhibit antiviral T cells ("veto" them) when the AKR.H-2^b cells are recognized. Consistent with this model, inhibition by AKR.H-2^b modulator cells was MHC restricted, and resulted in approximately a 10- to 70-fold decrease in the in vitro expansion of pCTL/CTL. Both CD8⁺ CTL and CD4⁺ Th responder cells were susceptible to inhibition by FasL⁺ AKR.H-2^b inhibitory cells as the basis for inhibition. The CTL response in the presence of inhibitory cells could be restored by several cytokines or agents that have been shown by others to interfere with activation-induced cell death (e.g., interleukin-2 [IL-2], IL-15, transforming growth factor , lipopolysaccharide, 9-cis-retinoic acid) but not others (e.g., tumor necrosis factor alpha). These results raise the possibility that this type of inhibitory mechanism is generalized as a common strategy for retrovirus infected cells to evade immune T-cell recognition.