Cepharanthine (Cc) is a biscoclaurine alkaloid extracted from Stephania cepharantha Hayata. In our previous study, Cc significantly enhanced thermosensitivity and thereby reduced thermotolerance in vitro, and intraperitoneal injection of Cc slightly enhanced thermosensitivity in vivo. In the present study, we investigated Cc's effect in vitro on the pattern of cell death after heating and the effect of intra-tumoral injection of Cc on in vivo thermosensitivity using a mouse fibrosarcoma, FSa-II, and C3H/He mice. Cc significantly enhanced the in vitro thermosensitivity of FSa-II cells with heating at 44degreesC, with increased Cc concentration. Time-lapse microscopic observation of individual cells confirmed that Cc treatment hastened both apoptosis (specifically, apoptotic budding) and necrosis (as indicated by staining with propidium iodide). Staining with annexin V-enhanced green fluorescent protein indicated that Cc used concomitantly with heating significantly increased the proportion of cells in the early stage of apoptosis. Cc combined with heating also significantly increased the proportion of cells with high intracellular caspase-3 activity, as detected by a substrate of caspase-3, PhiPhiLux-G(1)D(2). The intra-tumoral injection of Cc, followed by heating at 44degreesC, significantly delayed in vivo tumor growth, and this delay increased in a Cc concentration-dependent manner. Cc injected 30 min before heating delayed tumor growth more than Cc injected immediately before heating. These findings suggest the potential of Cc as a thermosensitizer to increase apoptosis of tumor cells.