['To investigate the radiosensitizing effects of cepharanthine (CEP) in the human cervical adenocarcinoma HeLa cell line and to examine the underlying mechanisms.', 'Survival of HeLa cells after treatment with or without ionizing radiation (IR) and CEP administration was investigated. MTT assays and apoptosis analysis were used to assess cytotoxicity. Nude mouse xenografts were established to evaluate the antitumor effects of CEP and IR in vivo. Expression of signal transducer and activator of transcription 3 (STAT3) and its downstream signaling molecules as well as cyclooxygenase-2 (COX-2) were examined by Western blot analysis.', 'Clonogenic assays showed that treatment with CEP and IR resulted in significant radiosensitization. Cepharanthine and IR treatment achieved maximum cytotoxic effects on HeLa cells with regard to apoptosis induction. Cepharanthine efficiently decreased IR-induced STAT3 and COX-2 activation. The STAT3 target genes, including the antiapoptotic Bcl-2 and the cell cycle regulator c-Myc, were decreased concomitantly. In vivo administration of CEP (20 mg/kg every 2 days) combined with radiation in HeLa xenografts enhanced tumor growth delay and apoptosis (indicated by activated caspase-3 Western blot analysis), with reduced expression of STAT3, Bcl-2, c-Myc, and COX-2.'] Is inhibition of signal transducer and activator of transcription 3 and cyclooxygenase-2 involved in radiosensitization of cepharanthine in HeLa cells?