A low concentration of differentiation inducers greatly enhances the in vitro and in vivo antiproliferative effects of interferon (IFN)alpha in several human cancer cells. Among the differentiation inducers tested, the sensitivity of cancer cells to IFNalpha was most strongly affected by cotylenin A. Cotylenin A, which is a novel fusicoccane diterpene glycoside with a complex sugar moiety, affected the differentiation of leukemia cells that were freshly isolated from acute myelogenous leukemia patients in primary culture. Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and its receptor DR5 were early genes induced by the combination of cotylenin A and IFNalpha in carcinoma cells. Neutralizing antibody to TRAIL inhibited apoptosis, suggesting that cotylenin A and IFNalpha cooperatively induced apoptosis through the TRAIL signaling system. Combined treatment preferentially induced apoptosis in human lung cancer cells while sparing normal lung epithelial cells. In an analysis of various cancer cell lines, ovarian cancer cells were highly sensitive to combined treatment with cotylenin A and IFNalpha in terms of the inhibition of cell growth. This treatment was also effective toward ovarian cancer cells that were refractory to cisplatin, and significantly inhibited the growth of ovarian cancer cells as xenografts without apparent adverse effects. Ovarian cancer cells from patients were also sensitive to the combined treatment in primary cultures. Combined treatment with cotylenin A and IFNalpha may have therapeutic value in treating human cancers including ovarian cancer.