In the Lewis rat, the encephalitogenic determinant of myelin basic protein (MBP), residues 68 to 88, induces an alpha beta + T cell population whose TCR beta-chains are exclusively derived from the V beta 8 TCR gene family. As presented here, sequencing of these beta-chains has revealed the following. 1) There is an absolute restriction to a single V beta 8 family member, previously identified as V beta 8.2. This V region is used by only 10% of the V beta 8+ TCR found in normal unprimed mesenteric and cervical lymph node T cell populations. 2) There is a serine at residue 97 (in the CDR3 region of the beta-chain) which appears to be Ag-specific and is not found in normal populations of adult T cells. 3) There is a size restriction of these MBP-specific beta-chains, resulting from the addition and deletion of nucleotides in the CDR3 region, which tend to cancel each other out. 4) There is a paucity of N-region nucleotide additions in the J region of these MBP-specific beta-chains. Such a reduced number of nontemplate-added nucleotides has been associated with receptors that rearrange early during development and fail to add nucleotides due to a lack of terminal deoxynucleotidyl transferase at that time. These results have led us to propose that the selection of MBP-reactive autoimmune T cells is based on both the Ag and the time frame when these cells are generated and enter the peripheral T cell pool.