Three treatments with 2,4 dioxo hexahydro triazine (DHT) significantly reduced the concentration of potato virus X (PVX) in systemically infected tobacco plants. In hypersensitive plants DHT caused a reduction in the number of local lesions produced by PVX. In systemic and hypersensitive hosts, treatment with DHT resulted in a more or less marked reduction in the concentration of, and in the number of local lesions caused by, potato virus Y (PVY), potato virus A (PVA), tobacco mosaic virus (TMV), and cucumber mosaic virus (CMV). Three days after treatment with DHT the incorporation of 32P into TMV RNA was significantly reduced, while that into host RNA was less affected.