Methyl salicylate, a volatile liquid, also known as oil of wintergreen, is made by a number of plants(1-9). Here we show that methyl salicylate is a major volatile compound produced by tobacco plants inoculated with tobacco mosaic virus. Methyl salicylate is synthesized from salicylic acid, a mon-volatile chemical signal required for the establishment of acquired resistance(10) and local and systemic induction of antimicrobial pathogenesis-related proteins(11). Methyl salicylate acts by being converted back to salicyclic acid. We conclude that methyl salicylate may function as an airborne signal which acitvates disease resistance and the expression of defence-related genes in neighbouring plants and in the healthy tissues of the infected plant.