We reported earlier that variable expression of the Epstein-Barr virus (EBV) encoded membrane protein LMP 1 in nasopharyngeal carcinoma and the host-cell-phenotype-dependent activity of the BCR2 promoter (one of the possible initiator sites for transcripts of Epstein-Barr nuclear antigens) in Burkitt's lymphoma (BL) lines can be related to the methylation status of the 5'-flanking regulatory regions of the BNLF 1 and BCR2 promoter, respectively. Here we report that clones of the BL line Mutu that differ in expression of LMP 1 also show a differential methylation pattern of the LMP 1 regulatory sequences: this region is hypomethylated in an LMP 1 expressing (group III) clone but methylated in a group I clone that does not express LMP 1. We introduced in vitro methylated reporter plasmids carrying BNLF 1 and BCR2 enhancer-promoter sequences into the BL line Raji and found that overall methylation of 5'-CG-3' sequences by the Spiroplasma methylase Sssl significantly reduced their activity compared to unmethylated or mock-methylated controls. Methylation of 5-CCGG-3' sequences by Hpall methyltransferase gave similar results. On the contrary, methylation of 5'GCGC-3' sequences by Hhall methyltransferase gave similar results. On the contrary, methylation of 5'-GCGC-3' sequences by Hpal methyltransferase resulted only in a moderate reduction of BNLF 1 enhancer-promoter activity. These data support the notion that methylation at discrete sites within control regions of latent, growth-transformation associated EBV genes may contribute to silencing their expression.