All multiple myeloma cell lines examined showed constitutively active I.kappa.B kinase (IKK), I.kappa.B.alpha. phosphorylation and constitutively active NF-.kappa.B. Curcumin, a chemopreventive agent, suppressed constitutive I.kappa.B.alpha. phosphorylation through inhibition of IKK activity and downregulated NF-.kappa.B. Curcumin also downregulated expression of NF-.kappa.B-regulated gene products such as I.kappa.B.alpha., Bcl-2, Bcl-x.sub.L, cyclin D1 and interleukin-6. Consequently, curcumin suppressed multiple myeloma cell proliferation and arrested cells at the G1/S phase of the cell cycle. Curcumin also induced apoptosis and chemosensitivity to vincristine. Overall, results presented herein provide a molecular basis for the treatment of multiple myeloma patients with this pharmacologically safe agent.