The Epstein-Barr virus (EBV) is a ubiquitous Herpes virus which causes infectious mononucleosis and is associated with such different neoplasms as Burkitt's lymphoma and nasopharyngeal carcinoma. EBV latently infects its target cells; nevertheless, evidence of viral replication in NPC tumours has been uncovered. Among the EBV transactivators, the ZEBRA protein plays a crucial role in switching the virus from a latent to a productive mode. ZEBRA protein was produced using a eukaryotic expression vector: the open reading frame containing the BZFLI cDNA has previously been inserted downstream from the adenovirus major late promoter leading to expression of a 38-kDa nuclear protein. We performed serological studies by employing ZEBRA protein expressed in human cells for immunofluorescence and Western-blot assays. We were able to detect IgG anti-ZEBRA antibodies (IgG/ ZEBRA) in 87% of NPC patients. These antibodies were absent in control sera; lgG/ZEBRA antibodies can be proposed as a useful marker for diagnosis of NPC tumors. Y.C., CHEN, J.Y., GI.ASER, K. and HENLE, W . , Frequency and levels of antihodies to Epstein Barr virub apccific DNase are elevated in ment of heterogeneous viral DNA. Pror. nut. Arud. Sri. (Wush.), 82, 40854089 (1985). CHEVALLIER-GRECO, A , , MANET, E., CHAVRIER, P., MOSNIER, C . , DAIL-LIE, J. and SERGEANT, A,, Both Epstein-Barr virus (EBV)-encoded transacting factors, EB 1 and EB2, are required to activate transcription from an EBV early promoter. EMBO J . , 5, 3243-3249 (1986).