Study of the antiviral action of the abnormal nucleoside analogue 2-(beta-D-ribofuranosyl)-5-amino-1,2,4-triazine-3(2H)-one was performed with Epstein-Barr virus (EBV) positive human B-cells (Raji) superinfected by EBV, as an acute infection model, and EBV-producing tamarin (Saguinus oedipus) B-cell line (B95-8), as a chronic infection model. The antiviral activity of the compound was determined by RT-PCR. It was shown that the IC₅₀ values (concentration of compound that inhibits the accumulation of the viral genome at 50 %) for 2-(beta-D-ribofuranosyl-5-amino-1,2,4-triazine-3(2H)-one were 0.5 ug/ml for the acute infection of EBV infected Raji cells and 3.8 ug/ml for the model of chronic infection. The anti-EBV action of Ganciclovir (the official anti-herpetic drug) was studied for comparison. IC₅₀ values for Ganciclovir were 1.0 ug/ml for the acute model and 6.2 ug/ml for the chronic model of EBV infections. Thus abnormal nucleoside 2-(beta-D-ribofuranosyl)-5-amino-1,2,4-triazine-3(2H)-one is a promising anti-EBV compound deserving further research.