The demyelination process that occurs in the central nervous system (CNS) of patients with multiple sclerosis (MS) is in part due to an inflammatory response in which CD4+ and CD8+ T cells and macrophages infiltrate white matter. In this study, we have identified a peptide sequence derived from the CNS-specific myelin protein proteolipid protein (PLP) which could bind to HLA-A3 and induce a HLA-A3-restricted CD8+ CTL response from HLA-A3+ donors. These PLP peptide-specific CTL could lyse HLA-A3+ target cells pulsed with a homologous peptide derived from the CRM1 protein of Saccharomyces cerevisae. These findings demonstrate the immunogenic potential of a PLP-derived peptide for generation of autoreactive HLA-A3-restricted CD8+ CTL, and further show that these CTL can be activated by a peptide derived from a common environmental microorganism.