Inoculation of tobacco cv. Xanthi nc or bean plants with the mixtures of benzothiadiazole (Bion) and tobacco mosaic tobamovirus (TMV) or alfalfa mosaic virus (AIMV), respectively did not show any inhibition of the number and size of the local lesions. Protective treatment of plants with Bion caused a significant decrease in disease incidence. In the case of tobacco cv. Xanthi nc and TMV or bean plants and AIMV that protective effect increased day by day and 6-7 days after treatment the production of local lesions was inhibited almost completely. Bean plants treated with Bion demonstrated resistance ranging between 60-90% also in nontreatcd parts. Bean and tomato plants pretreated with O.Ol% Bion were effectively (in 60-70%) protected against systemic infection by tomato black ring ncpovirus (TBRV).