The effect of (E)-5-(2-bromovinyl)- and 5-vinyl-1-beta-D-arabinofuranosyluracil (BrVaraU, VaraU) in comparison to 9-(2-hydroxyethoxymethyl)guanine (ACV) on the proliferation of human lymphoblastoid P3HR-1 cells in culture and on the expression of Epstein-Barr virus capsid antigen (VCA) in the same cells was evaluated. After 7 days of cell growth, at 100 mumol/l the total number of new generations in drug-treated cultures was similar or 5 and 10% below that in drug-free control cultures, for VaraU, ACV, and BrVaraU, respectively. During the same time the percentage of VCA-expressing cells decreased from 6.3% in drug-free cultures to 1.3, 1.5, and 2.0% in cultures treated with VaraU, ACV and BrVaraU, respectively. In VaraU-treated cultures a further decrease in the percentage of VCA-positive cells down to 0.5% was revealed 7 days after drug removal. VaraU was also effective in reducing the proportion of VCA-expressing cells at 10 and 1 mumol/l. At 14 days after drug removal, the inhibitory effect of ACV was nearly reversed, whereas BrVaraU showed a prolonged VCA- suppressing effect.