A new monoclonal antibody has been used to examine immunohistochemically the expression of the Epstein-Barr virus (EBV)-encoded nuclear antigen (EBNA) 1 in virus-associated epithelial lesions. EBNA1 was detected in the tumour cell nuclei of 10/13 undifferentiated nasopharyngeal carcinomas and of 10/10 EBV-associated gastric carcinomas. EBNA1 was also detected in 13 of 16 oral hairy leukoplakia (HL) samples, where its expression was confined to nuclei in the upper epithelial cell layers whilst basal epithelial cells were negative. This observation is in agreement with previous studies demonstrating the absence of latent EBV infection in the basal cell compartment of HL and suggests an essential role for EBNA1, not only in latent EBV infection but also in virus replication.