Although arsenic trioxide (As(2)O(3)) induces apoptosis in a relatively wide spectrum of tumors, the sensitivity of different cell types to this treatment varies to a great extent. Because reactive oxygen species (ROS) are critically involved in As(2)O(3)-induced apoptosis, we attempted to explore the possibility that elevating the cellular ROS level might be an approach to facilitate As(2)O(3)-induced apoptosis. Emodin, a natural anthraquinone derivative, was selected because its semiquinone structure is likely to increase the generation of intracellular ROS. Its independent and synergistic effects with As(2)O(3) in cytotoxicity were studied, and the plausible signaling mechanism was investigated in HeLa cells. Cell Proliferation Assay and flow cytometry were used to assess cell viability and apoptosis. Electrophoretic mobility shift assay, luciferase reporter assay, and Western blotting were performed to analyze signaling alteration. The results demonstrated that coadministration of emodin, at low doses of 0.5-10 micro M, with As(2)O(3) enhanced As(2)O(3)-rendered cytotoxicity on tumor cells, whereas these treatments caused no detectable proproliferative or proapoptotic effects on nontumor cells. ROS generation was increased, and activation of nuclear factor kappaB and activator protein 1 was suppressed by coadministration. All enhancements by emodin could be abolished by the antioxidant N-acetyl-L-cysteine. Therefore, we concluded that emodin sensitized HeLa cells to As(2)O(3) via generation of ROS and ROS-mediated inhibition on two major prosurvival transcription factors, nuclear factor kappaB and activator protein 1. This result allows us to propose a novel strategy in chemotherapy that uses mild ROS generators to facilitate apoptosis-inducing drugs whose efficacy depends on ROS.