The proteasome inhibitor bortezomib has been applied successfully to treat multiple myeloma (MM). Its synergistic effects with other anticancer drugs have been studied widely. In the present study, it was found that lidamycin (LDM), a member of the enediyne antibiotic family, showed much more potent cytotoxicity than bortezomib to MM cell lines: U266 and SKO-007. Here, we investigated the potential synergy of bortezomib and LDM on MM cells. The results showed that cotreatment of bortezomib and LDM synergistically induced cytotoxicity and apoptosis in MM cell lines, followed by enhanced caspase-3 cleavage and degradation of poly-ADP-ribose polymerase together with the decreased nuclear factor-κB protein. These two drugs synergistically induced apoptosis, which was associated with enhanced activation of two mitogen-activated protein kinases: p38 mitogen-activated protein kinase and c-Jun NH(2)-terminal kinase. Moreover, bortezomib plus LDM synergistically induced apoptosis was also associated with downregulation of extracellular signal-regulated kinase, and induction of endoplasmic reticulum stress response. Overall, our results indicate that the combined regimen of bortezomib and LDM might be a potential therapeutic remedy for the treatment of MM.