In EBV-immortalized lymphoblastoid cell lines (LCLs) a small number of “latent” proteins are expressed. These are the EBV nuclear antigens, EBNAs 1–6, and a latent membrane protein, LMP. We have investigated the expression of these proteins in a variety of EBV-associated tumours and cell lines. Whereas transplant and B-cell lymphomas from cottontop tamarins appear to express the full range of antigens found in LCLs, we and others have found that in Burkitt's lymphomas (BL) and a nasopharyngeal carcinoma (NPC) isolate, EBNA expression is restricted to EBNA-1. (In NPC, but not in BL, LMP may also be expressed.) In order to ask what restricts the expression of EBNA 2–6 in NPC and BL cells it seemed reasonable to consider the possibility that the DNA sequences normally regulating expression of these antigens could be chemically modified. In this analysis, a tight inverse correlation between methylation of CpG dinucleotides in the 5′ flanking region of the EBNA-2 gene and the expression of EBNAs 2–6 has been revealed. In the NPC tumour, CpG methylation within the gene is also observed, as is specific methylation over the EBNA-1 region I and II binding sites (in oriP). The significance of these observations is considered.