The use of NF-.kappa.B inhibitors to enhance the cytotoxic effects of chemotherapy or radiation therapy in the treatment of neoplastic conditions is described. When chemotherapeutic compounds and radiation activate NF-.kappa.B within cells, the cells are more resistant to the cytotoxic effects of these treatments. Inhibition of NF-.kappa.B activation is disclosed to provide a method to improve the efficacy of these cancer therapies.