Interleukin - 6 - dependent mouse hybridoma cell line KD83 used to test the biologic activity of interleukin-6 in synovial fluid from 37 patients with temporomandibular disorders. The results showed taht the interleukin-6 level was greater than 100 U/mL in 13 of 18 patients with degenerative joint disease and in five of 12 patients with temporomandibular disc displacement. However, the interleukin-6 level was less than 100 U/mL (range, 20 to 75 U/mL) in all patients with masticatory muscle disorder. It has been found that degenerative joint disease tends to have acute and chronic stages, and interleukin-6 activity was probably related to the acute stage in the patients. Histologic studies of the synovium from several patients with degenerative joint disease showed a variable degree of hyperplasia of the synovial lining cells and chronic inflammation in five of eight specimens. Immunostaining studies clearly showed the presence of significantly more HLA-DR-expressing cells (human leukocyte antigen-D-related) in synovium. Although it is unlikely that immune responses play an important primary role in initiating synovial inflammation and cartilage destruction, immune reactions may be one important factor in the maintenance and severity of some patients with temporomandibular disorders.