To analyze the nature of the B cell response in the synovial tissue of a patient with rheumatoid arthritis (RA). Specifically, we sought to determine if the pattern of immunoglobulin expression was consistent with polyclonal stimulation of B cells or an antigen-driven response. We generated an unrestricted complementary DNA (cDNA) library from the diseased synovium of a rheumatoid factor (RF)-positive patient with an 18-year history of RA. A random sample of kappa light chain recombinants was identified, and sequence analysis was performed. The variable domains were compared with an extensive database of germline and cDNA kappa sequences. We found a light chain repertoire enriched for kappa transcripts containing 2 V kappa gene segments (Humkv325 and Humkv328) that are frequently associated with paraproteins expressing RF activity. Kappa variable domains from synovium contained numerous somatic mutations which resulted in frequent replacement of amino acids that encode the classic antigen-binding site. Unexpectedly, many of these kappa transcripts contained non-germline-encoded nucleotides (N regions) at the site of V kappa-J kappa joining. The combination of N-region addition and variation in the sites of V kappa-J kappa splicing generated unusually long complementarity-determining region 3 regions and charged amino acids near the V kappa-J kappa splice site. The pattern of somatic mutations found in this patient sample supports the hypothesis that these synovium-derived plasma cells are the product of immunoglobulin receptor-dependent (i.e., antigen-driven) selection. The extent of N-region addition raised the additional possibility that these antibodies derive from an unusual set of B lymphocytes that have escaped normal regulation.