Rheumatoid arthritis (RA) is characterized by inflammation of synovium, in which immunoglobulin-secreting plasma cells are generally present. The forces driving immunoglobulin expression in RA synovium are unknown. Sequences of VH and Vκ transcripts from an RA synovial cDNA library demonstrate patterns of somatic mutation typical of an antigen-driven response. Moreover, 5% of the κ repertoire appears to derive from the same B cell progenitor, suggesting an oligoclonal response. Immunoglobulin expression in this synovium thus appears to result from antigen stimulation. In addition, this patient's synovium is enriched for unusually long Vκ-Jκ joins (CDR3s), suggesting abnormal selection or regulation of the B cell response in RA.