Objective: To study apoptosis induced by radiation in two nasopharyngeal carcinoma (NPC) cell lines, CNE and CNE-2. Methods: Hoechst 33342 staining, immunohistochemical staining, RT-PCR, DNA dot blotting and South blotting were used to identify apoptosis.Results: A single dose of X-irradiation resulted in apoptosis, the apoptotic index (AI) was time- and dosedependent. Different apoptotic responses existed in the two cell lines. Immunohistochemical staining showed that bcl-2 protein was strongly positive in CNE but negative in CNE-2. However, RT-PCR revealed p53mRNA in CNE-2 but not in CNE. P53 and bcl-2 genes were both present in the two cell lines as shown by DNA blotting, but the 2.8 kb fragment of the p53 gene was much lower than the 5.6 kb fragment on CNE which was clearly shown in South hybridization, suggestive of partial deletion of p53 gene in CNE. Conclusion:Apoptotic response to radiation is different in two NPC cell lines. CNE is more radioresistant than CNE-2.Overexpression of bcl-2 protein and partial deletion of p53 gene may explain their difference in radiosensitivity.