We demonstrated that RL male 1-specific cytotoxic T lymphocytes (CTL) were generated in spleen cells from the tumor-rejected CB6F1 mice on in vitro stimulation with Ld-binding pRL1a peptide. We showed that CD4 T cells and antigen-presenting cells were necessary for CTL generation. Furthermore, CTL generation was inhibited by the addition of anti-I-Ad mAb, but not anti-I-Ek/d mAb, to the culture. However, no binding of the pRL1a peptide to the I-Ad molecule could be demonstrated. No inhibition by the pRL1a peptide of I-Ad-restricted IL-2 production by ovalbumin (OVA)-specific CD4 T cell hybridoma DO-11.10 was observed on stimulation with the specific OVA peptide. Moreover, no specific proliferative response or IL-2 production was observed with CD4 T cells in spleen cells from the tumor-rejected mice on stimulation with a range of mitomycin C-treated RL male 1 cells, or with RL male 1 lysate or the pRL1a peptide. Rather, the activation of autoreactive CD4 T cells and the IL-2 production from them were observed. CD4 T cells from CB6F1 mice that had rejected other tumors such as EL4 or MOPC-70A also helped the generation of pRL1a-specific CTL. These findings suggested the involvement of autoreactive CD4 T cells in CTL generation against the pRL1a peptide.