Previous studies investigating the role of Epstein-Barr virus (EBV) in undifferentiated nasopharyngeal carcinoma (NPC) have been performed on extracts from biopsies. The authors analyzed expression and localization of viral gene products in 18 undifferentiated NPCs at the cellular level using immunohistology and in situ hybridization. All cases were EBV-positive. The small nuclear EBV-encoded RNAs, EBERs, were regularly expressed whereas the latent membrane protein, LMP1, and EBV was detectable only in four cases (22%) and the nuclear antigen 2 was not detectable. The BZLF-1 protein of EBV which disrupts viral latency, was not detectable, confirming that the virus is latent in the tumor cells. Although the expression of the CD23 antigen in transplantable NPCs has been reported, our study demonstrates that expression of this antigen in human undifferentiated NPCs is rare. In contrast, almost all cases expressed the CDw70 antigen. Since in normal tissues this antigen is present only in activated lymphoid blasts, this finding may be relevant for the differential diagnosis of undifferentiated NPCs.