Immunodominant T cell epitopes of myelin basic protein (MBP) may be target antigens for major histocompatibility complex class II-restricted, autoreactive T cells in multiple sclerosis (MS). Since susceptibility to MS is associated with the DR2 haplotype, the binding and presentation of the immunodominant MBP(84-102) peptide by DR2 antigens were examined. The immunodominant MBP(84-102) peptide was found to bind with high affinity to DRB1*1501 and DRB5*0101 molecules of the disease-associated DR2 haplotype. Overlapping but distinct peptide segments were critical for binding to these molecules; hydrophobic residues (Val189 and Phe92) in the MBP(88-95) segment were critical for peptide binding to DRB1*1501 molecules, whereas hydrophobic and charged residues (Phe92, Lys93) in the MBP(89-101/102) sequence contributed to DRB5*0101 binding. The different registers for peptide binding made different peptide side chains available for interaction with the T cell receptor. Although the peptide was bound with high affinity by both DRB1 and DRB5 molecules, only DRB1 (DRB1*1501 and 1602) but not DRB5 molecules served as restriction elements for a panel of T cell clones generated from two MS patients suggesting that the complex of MBP(84-102) and DRB1 molecules is more immunogenic for MBP reactive T cells. The minimal MBP peptide epitope for several T cell clones and the residues important for binding to DRB1*1501 molecules and for T cell stimulation have been defined.