Understanding the immune response to myelin antigens in regard to the peptide/MHC/TCR complex is important in defining pathogenesis of demyelinating autoimmune diseases and in developing antigen-specific therapies. We previously reported that individual multiple sclerosis patients may use certain dominant TCR V beta chains to recognize immunodominant MBP peptides. In examining the TCR beta chain usage, we observed repeated TCR VDJ sequences among different T-cell lines isolated from the same patient. This suggested that a few expanded T-cell clones may dominate the immune response to immunodominant MBP peptides. Here, we report experiments where TCR rearrangements were used as a probe for the clonal origin of MBP specific T-cells cultured from blood lymphocytes of MS patients and normal subjects. In two patients with the DR2 haplotype that were analyzed in detail, the T-cell response to MBP was focused on the MBP (84-102) peptide and in vivo expanded population(s) dominated the response to the MBP (84-102) peptide. Two MBP (84-102) specific T-cell clones from a normal subject with the DR2 haplotype were also found to have identical TCR sequences. Clonality was proven by demonstrating that independent clones had identical TCR alpha and beta chain sequences as well as identical sequences of a TCR gamma chain or of a second TCR alpha chain rearrangement. These data suggest that the response to human MBP is dominated in at least some subjects by expanded clones that may persist in vivo for relatively long periods of time.