With the MTT tetrazolium WEHI 164 clone 13 cell cytotoxicity assay, we measured TNF alpha (tumor necrosis factor) activity in synovial fluids of TMJDS (Temporomandibular joint dysfunction syndrome). We found no detected TNF alpha level from 5 patients with muscle dysfunction, raised TNF alpha levels from 5 of 11 patients with internal derangement and from 9 of 11 patients with organic destruction (osteoarthritis). The findings of biologically active TNF alpha in synovial fluids of TMJDS suggest that TNF alpha may play a role in the pathogenesis of TMJDS.