Resistance of melanoma cells to chemotherapeutics remains a major obstacle to successful treatment of melanoma once it has spread beyond locoregional sites. We report in this study that activation of the unfolded protein response (UPR) is involved in resistance of melanoma cells to two chemotherapeutic drugs, cisplatin (CDDP) and adriamycin, and this is associated with glucose-regulated protein 78 (GRP78)-mediated inhibition of activation of caspase-4 and -7. The UPR was constitutively activated in cultured melanoma cell lines and fresh melanoma isolates as evidenced by elevated expression levels of the GRP78 protein and the active form of x-box-binding protein 1 messenger RNA. Treatment with CDDP or adriamycin further increased the levels, indicative of induction of endoplasmic reticulum stress and activation of the UPR by the drugs. Inhibition of GRP78 by small-interference RNA (siRNA)-sensitized melanoma cells to CDDP- and adriamycin-induced apoptosis. This was associated with enhanced caspase-4 and -7 activation as siRNA knockdown of the caspases blocked induction of apoptosis. In contrast, overexpression of GRP78 attenuated activation of caspase-4 and -7 and induction of apoptosis by the drugs. CDDP- and adriamycin-induced activation of caspase-4 and -7 appeared to be mediated by calpain activity in that it was blocked by the calpain inhibitors calpeptin and PD150606 even when GRP78 was inhibited by siRNA. These results provide new insights into resistance mechanisms of melanoma cells to CDDP and adriamycin and identify GRP78 as a potential target for enhancing chemosensitivity in melanoma.