H-2b mice are immunologic responders to the tumorigenic MCF1233 murine leukemia virus (MuLV), an AKV-related virus derived from endogenous C57BL MuLV. We have identified an immunodominant CTL epitope that is expressed on MCF1233 MuLV-induced lymphomas of H-2b mice. C57BL/10 (B10) mice were immunized with an MCF1233-induced B10 B cell lymphoma, and tumor-specific CTL cultures were generated in vitro. These were tested for recognition of synthetic class I-binding MuLV peptides, selected for class I allele-specific motifs. One of 28 candidate peptides sensitized target cells for CTL recognition. This peptide seems to be an immuno-dominant epitope, because it was recognized by all independent CTL clones, isolated from the tumor-specific bulk culture. The epitope (KSPWFTTL) is derived from the MCF1233 MuLV envelope (env)-p15E region and is shared by all endogenous AKV types of MuLV. It has an optimal length of eight amino acids and is presented by the Kb H-2 class I molecule. Interestingly, Friend, Moloney, and Rauscher (FMR) types of MuLV are not recognized by MCF MuLV-directed CTL. The FMR env-p15E proteins have a single amino acid difference at the first position of the MCF1233 MuLV epitope (RSPWFTTL instead of KSPWFTTL). The corresponding FMR-encoded peptide bound class I H-2 Kb equally well as the MCF peptide, but it was poorly recognized by MCF1233 MuLV-specific CTL. Moreover, in the Rauscher MuLV-induced cell line RMA the FMr peptide seems not to be processed for recognition by CTL, which was illustrated by experiments with CTL elicited against this peptide. Altered TCR interaction as well as lack of processing thus may explain the type specificity of MCF1233 MuLV-directed CTL.