Tomato plants (Lycopersicon esculentum Mill.) inoculated with tomato white necrosis virus (VNBT) and treated with Virazole (1-beta-D-ribofuranosyl--1, 2, 4-triazole-3-carboxamide) at a concentration of 500 mg/l developed systemic virus symptoms in only 40 per cent of the plants in which a remarkable reduction in virus concentration was also observed. From inoculated and Virazole-treated plants which had produced no symptoms, no virus could be recovered. This result suggests that Virazole may inhibit replication of VNBT in tomato.