['To determine the role of lymphocyte proliferation assay of synovial fluid mononuclear cells (SFMC) with whole fraction bacteria in the diagnosis of reactive arthritis (ReA) or arthritis of unknown origin.', 'We stimulated SFMC of 52 unselected patients who consecutively presented in our rheumatology outpatient clinic with the following diagnoses: ReA (n = 8), rheumatoid arthritis (RA) (n = 16), ankylosing spondylitis (AS) (n = 6), osteoarthritis (OA) (n = 5), psoriatic arthritis (PsA) (n = 5) and arthritis of varying origin (AVO) (n = 12) and peripheral blood MC (PBMC) of 10 healthy controls with arthritogenic (Y. entero-colitica, S. enteritidis, C. trachomatis) and non-arthritogenic (E. coli, K. pneumoniae, S. pyogenes, C. albicans) bacteria/mitogens and Tetanus toxoid. T cell proliferation was measured in a standard [3H] Thymidine uptake assay.', 'In all groups of patients tested, SFMC could be stimulated both by arthritogenic and non-arthritogenic bacteria. So-called specific responses were observed in patients with ReA, but also in RA and AS.'] Is synovial fluid lymphocyte proliferation in response to crude microbial antigens useful as a diagnostic test to specifically indicate a bacterial cause of arthritis?