T lymphocytes have been implicated in the pathogenesis of rheumatoid arthritis. Interestingly, many of the activated T cells isolated from the synovial fluid of individuals with rheumatoid arthritis react with antigens from Mycobacterium tuberculosis or BCG. This response is seen to a much lesser extent in the peripheral blood of these patients. To investigate the nature of the T-cell response to BCG in RA, we isolated T cells from the synovial fluid of a patient with early-stage rheumatoid arthritis, stimulated them with BCG and cloned by limiting dilution. Staining with monoclonal antibodies specific for different V beta gene families revealed a statistically significant greater proportion of synovial-derived T-cell clones expressing the V beta 8 gene family product compared with peripheral blood clones. While the antigen specificity of some of the clones could not be determined, several of the clones displayed distinct antigen reactivities. Sequencing the TCR beta chain genes of these T cells suggested that although the V beta 8 gene products appeared to be over-represented in these BCG-specific clones, each clone utilized distinct J beta gene segments and used N segment addition to different extents. In addition, no common motifs were identified in the beta chain CDR3s of the clones sequenced. Analysis of bulk cultured BCG-specific SF T cells and unstimulated peripheral blood T cells for V beta 8 gene expression also revealed a large amount of diversity within the CDR3 region. Thus, the T-lymphocyte response to BCG in this patient with early rheumatoid arthritis appears to be quite heterogeneous.