An immediate-early protein of murine cytomegalovirus (MCMV), pp89, elicits an immunodominant and protective major histocompatibility complex (MHC) class I Ld-restricted CD8+ T-lymphocyte response. Remarkably, presentation of the naturally processed peptide of pp89, the nonapeptide YPHFMPTNL, is abolished during permissive MCMV infection in vitro. This defect in pp89 presentation is due to the expression ofMCMV early gene functions that specifically block the transport of peptide-charged MHC class I complexes to the cell surface (M. Del Val, H. Hengel, H. Hacker, U. Hartlaub, T. Ruppert, P. Lucin, and U. H. Koszinowski, J. Exp. Med. 176:729-738, 1992). Here, we demonstrate that MCMV-specific CD8+ T lymphocytes can reconstitute pp89 presentation in a parakrine fashion. The lymphocytes mediate the restoration of antigen presentation by MCMV-infected cells by releasing gamma interferon (IFN--y). IFN--y has no effect on synthesis and stability of the viral antigen pp89 nor does it interfere with the expression of viral early genes and their inhibitory effect on MHC class I molecule maturation. IFN--y results in a 25-fold increase in the synthesis of MHC class I molecules and a similar increase in the abundance of pp89-derived peptide. Many of the MHC molecules remain retained by the viral effect, but a surplus of MHC molecules escapes the effect and provides the effective surface presentation of the peptide. Adoptive cell transfer studies demonstrate the IFN-'y dependence of CD8+ T-lymphocyte function in vivo. Altogether, these data reconcile the paradoxical findings of an impaired pp89 presentation in vitro in parallel with pp89-specific CD8+ T-cell protection in vivo. The results also imply a role of IFN-y in the T-lymphocyte-mediated control of cytomegalovirus infection. The known propensity of cytomegalovirus to cause serious disease in the immunocompromised host is discussed in the light of these findings.