A viral aetiology for rheumatoid arthritis and inflammatory connective tissue diseases has been sought in general terms first, by studying viral growth patterns in lymphocytes from the blood and lesions of patients affected second, by analysing lymphocyte concentrations of the interferon-induced enzyme 2-5 oligo-adenylate synthetase (2-5 A); and third, by probing Southern blots of lymphocyte DNA with viral probes. Indirect evidence consistent with a viral aetiology has been found in several such diseases, but direct proof has been difficult to adduce. There is some suggestion that herpes simplex viral (HSV) DNA is present in Behcet's blood lymphocytes, but the findings are inconsistent. It is also plausible that viruses such as HSV do not induce these diseases through classic immunopathological mechanisms, but as "promoters" of abnormal lymphoproliferation in individuals with predisposing defects, possibly related to selective DNA repair defects.