To characterize the binding specificity and light- and heavy-chain variable region usage in monoclonal human autoantibodies (mAAbs) to T-cell receptors, we constructed heterohybridomas from peripheral blood B cells of three rheumatoid arthritis (RA) patients. From a panel of more than 200 heterohybridomas secreting IgM autoantibodies binding to T-cell receptor V beta chain first complementarity determining segments (CDR1), we characterized two IgM/lambda molecules from a single patient in detail. These bound to both CDR1 peptide epitopes and intact TCR of recombinant single-chain T-cell receptor constructs, and to T-cell surface TCR. Spectratype analysis using epitopes mimicking a set of 24 V beta genes indicated that one molecule bound only a few members of the set, whereas the second showed considerable epitope promiscuity by binding to more than half of the tested CDR1 peptides. Both mAAbs used variants of a V lambda 3 gene that were very similar to one another and to the germline gene. The epitope-promiscuous autoantibody used a V(H)4 gene identical to a germline prototype, while the other incorporated a V(H)3 sequence differing in only a single residue from its germline prototype. The CDR3s of both were large and distinct from each other as well as from the corresponding segments of rheumatoid factors and "cold agglutinins" using the same or related V-H germline genes. These mAAbs offer models for deciphering the basis of epitope promiscuity, and serve as candidates for direct use in immunomodulation because they are of intrinsic human origin and do not require molecular engineering to adapt them for use in therapy.