Humans frequently produce serum IgM autoantibodies reactive with T-cell receptor beta chains at a determinant defined by peptides corresponding to the first complementarity determining region. It is likely that this determinant serves as a public idiotype involved in immunoregulation. Following screening of culture supernatants from over 60 Epstein-Barr virus-carrying B-cell lines of normal and neoplastic origin, we identified a line, IARC307, that secretes an IgM kappa protein showing marked specificity for the V beta 8.1 CDR1 sequence CKPISGHNSLFQWYRQT. We cloned and sequenced the complete variable regions of the V kappa and VH chains used by the autoantibody. The light chain has a V kappa III sequence related to the 'a' subgroup and uses J kappa 2. The heavy chain has a VHIII sequence essentially identical to the germ-line sequence DP54 and uses the JH6C minigene. The CDR3 is unique, differing from those of other autoantibodies. The antibody is rigorously specific in its specificity for the V beta 8 peptide and does not show polyspecificity for protein or DNA antigens.