Interleukin (IL)-6-mediated signalling attenuates the anti-myeloma activity of glucocorticoids (GCs). We therefore sought to evaluate whether CNTO 328, an anti-IL-6 monoclonal antibody in clinical development, could enhance the apoptotic activity of dexamethasone (dex) in pre-clinical models of myeloma. CNTO 328 potently increased the cytotoxicity of dex in IL-6-dependent and -independent human myeloma cell lines (HMCLs), including a bortezomib-resistant HMCL. Isobologram analysis revealed that the CNTO 328/dex combination was highly synergistic. Addition of bortezomib to CNTO 328/dex further enhanced the cytotoxicity of the combination. Experiments with pharmacologic inhibitors revealed a role for the p44/42 mitogen-activated protein kinase pathway in IL-6-mediated GC resistance. Although CNTO 328 alone induced minimal cell death, it potentiated dex-mediated apoptosis, as evidenced by increased activation of caspases-8, -9 and -3, Annexin-V staining and DNA fragmentation. The ability of CNTO 328 to sensitize HMCLs to dex-mediated apoptosis was preserved in the presence of human bone marrow stromal cells. Importantly, the increased activity of the combination was also seen in plasma cells from patients with GC-resistant myeloma. Taken together, our data provide a strong rationale for the clinical development of the CNTO 328/dex regimen for patients with myeloma.