Object: To study the existent form of EBV genome in nasopharyngeal carcinoma (NPC) biopsies, in a transplanted NPC tumor SUNT-1 and its corresponding epithelial cell line SUNE-1. Methods: By using polymerase chain reaction (PCR) amplification of Epstein-Barr virus (EBV) BamHI W fragment, EBV DNA was detected in 20/20 biopsy specimens of poorlydifferentiated, as well as in a nude mouse xenografted NPC tumor (SUNT-1, from passage 1 to 34) and in the corresponding epithelial cell line (SUNE-1, from passage 1 to 62). The intracellular form of EBV genome was studied by analyzing the terminal structure using a LMP2A probe and an in situ lysing gel technique.Results: A single EBV fused terminal DNA fragment was detected in 19 biopsy specimens, two hybridized bands were seen in one specimen. These results indicate that an episomal form of EBV genome is predominantly present in most NPC biopsy specimens, but insertion of the genome into the host chromosome could not be excluded. Conclusion: The finding suggests that EBV infection precedes clonal amplification of transformed cells, or in a rare case, that a single EBV infected clone is predominant in the development of NPC. Linear form of EBV DNA was detected in the 20th passage of SUNE-1;this may imply the in vitro activation of the productive cycle of EBV.