Synovial sarcoma is a relatively common soft tissue sarcoma with an aggressive clinical course. Although surgery is currently the first treatment modality, improvement of adjuvant chemotherapy is deemed essential to improve the clinical outcome. Nuclear factor-kappaB (NF-kappaB) is constitutively activated in various cancer cells and has emerged as a potential therapeutic molecular target; however, the possible involvement of NF-kappaB in the pathology of sarcomas remains to be clarified. Herein we examined the effects of a novel NF-kappaB inhibitor, dehydroxymethylepoxyquinomicin (DHMEQ) on two synovial sarcoma-derived cell lines, HS-SY-II and SYO-1. The growth of both cell lines was completely inhibited by DHMEQ and apoptosis was induced at 10 microg/ml. Additionally, we found that DHMEQ showed additive effects when used in combination with other cytotoxic agents. These observations indicate that inhibition of NF-kappaB activity may serve as a potential therapeutic target for synovial sarcoma.