T cell responses to myelin basic protein (MBP) are potentially involved in the pathogenesis of multiple sclerosis (MS). Immunization with irradiated MBP-reactive T cells (T cell vaccination) induces anti-idiotypic T cell responses that suppress circulating MBP-reactive T cells. This T cell-T cell interaction is thought to involve the recognition of TCR expressed on target T cells. The study was undertaken to define the idiotypic determinants responsible for triggering CD8+ cytotoxic anti-idiotypic T cell responses by T cell vaccination in patients with MS. A panel of 9-mer synthetic TCR peptides corresponding to complementarity-determining region 2 (CDR2) and CDR3 of the immunizing MBP-reactive T cell clones were used to isolate anti-idiotypic T cell lines from immunized MS patients. The resulting TCR-specific T cell lines expressed exclusively the CD8 phenotype and recognized preferentially the CDR3 peptides. CDR3-specific T cell lines were found to lyze specifically autologous immunizing MBP-reactive T cell clones. The findings suggest that CDR3-specific T cells represented anti-idiotypic T cell population induced by T cell vaccination. In contrast, the CDR2 peptides were less immunogenic and contained cryptic determinants as the CDR2-specific T cell lines did not recognize autologous immunizing T cell clones from which the peptide sequence was derived. The study has important implications in our understanding of in vivo idiotypic regulation of autoimmune T cells and the regulatory mechanism underlying T cell vaccination.