Background: Synovia of patients with long-standing rheumatoid arthritis (RA) are typically infiltrated with B lymphocytes and plasma cells that secrete large amounts of immunoglobulin. The CDR3 of an immunoglobulin heavy chain is composed of the V(H)-D(H)-J(H) join, with interposed N region addition, and thus defines clonal relatedness. Furthermore, the CDR3 lies at the center of the antigen binding site, so its length and composition influence antigen binding. We sought definitive evidence of an antigen- driven B cell response (i.e., clones derived from the same V(H), D(H), and J(H) gene segments with share somatic mutations) in RA synovial mRNA transcripts, and to characterize CDR3 intervals at the target of inflammation in this autoimmune disease. Materials and Methods: We screened a cDNA library generated from unselected cells from the knee joint of a 62-year-old white female with long-standing RA. This technique does not have the potential bias of selecting for antibodies that express a particular reactivity such as rheumatoid factor. CΓ recombinants were sequenced and progenitor V(H), D(H), and J(H) gene segments were assigned and somatic mutations determined by comparison to germline sequences. Analyses of D(H) reading frame utilization and hydropathy characteristics of CDR3s were performed. Results: Two of 67 recombinants were derived from the same V(H) (V3-11) and J(H) gene segments, demonstrated shared mutations, and contained nearly identical V(H)- D(H)-J(H) joins, including N region addition. Three other recombinants contained identical sequence throughout the variable domain. We also found preferential utilization of a limited number of V(H) and D(H) gene segments and marked preference for a D(H) reading frame encoding predominantly hydrophilic residues. Conclusions: Analysis of expressed heavy chain variable domains strongly supports the hypothesis that the B cell response in RA synovium is at least in part antigen driven and oligoclonal.