Some rheumatoid arthritis (RA) synovia contain structures similar to germinal centers (GC), the site of affinity maturation of B lymphocytes [1,2]. Previous analyses of immunoglobulin (Ig) kappa and lambda light chains expressed in RA synovia showed clonally related sequences with frequent N region addition and unusually long CDR3s [3,4]. The presence of clonally related Ig heavy chain sequences in GC-like structures from RA synovia suggests in situ antigen-dependent B cell maturation. RAG1 and 2, enzymes that mediate V(D)J recombination during B cell development, are expressed in a subset of GC B cells in normal peripheral lymphoid organs [5,6]. RAG expression allows secondary Ig rearrangements, which salvages B cells with undesirable specificities or low antigen affinity (receptor revision). We sought to determine whether RAG and TdT (the enzyme responsible for N region addition) are expressed in RA synovia and whether secondary Ig rearrangements occur. Using nested RT-PCR, we detected RAG1, RAG2, and TdT mRNA in 8, 9, and 6 of 12 synovial samples (11 RA, 1 JRA), respectively. RAG1 was expressed in B cells (5/8 samples) and T cells (4/8). RAG2 was expressed in B cells (4/8) more often than in T cells (1/8). TdT was expressed in B cells only (2/8). Immunohistochemical staining indicated that RAG proteins were distributed in lymphoid aggregates. In some synovia, secondary rearrangement products (ds-DNA breaks at recombination signal sequences in the Jkappa region) were detected by ligation-mediated PCR. We speculate that receptor revision in nonlymphoid tissues such as RA synovia may generate autoreactive antibodies, which has important implications for chronic inflammatory diseases.