It has been proposed that plants express resistance to pathogens when the product of a resistance gene interacts with an elicitor molecule produced by the pathogen. Although there is one instance with tobacco mosaic virus (TMV) in which virus resistance is known to act through the same type of mechanism, it is not known whether this model accounts generally for resistance interactions with plant viruses. To address this issue the interactions of resistance genes in potato with potato virus X (PVX) have been analysed at the molecular level. PVX is an RNA virus that is affected by three different types of resistance locus in various potato cultivars. By using recombinant isolates of PVX, incorporating components of strains or mutant viruses able to overcome or avoid the effects of the resistance loci, we have identified different regions of the viral genome that determine the outcome of the resistance interaction. This information has allowed us to investigate the resistance in detail. For example, with the resistance specified by the Rx locus, it has been shown that the coat protein is an avirulence determinant and elicitor of an induced resistance. This resistance acts by reducing virus accumulation in the inoculated cell. Although the recognition component of the resistance is highly specific, the induced response is apparently non-specific and is effective against viruses unrelated to PVX in cells doubly inoculated with PVX and a second virus. The recognition function of Rx is also expressed in Gomphrena globosa which is a non-host plant of PVX. Based on these data, we propose that virus resistance fits the paradigm of resistance to fungal and bacterial pathogens and that there are similarities between the mechanism of cultivar specific resistance and non-host resistance to pathogen attack. Further analysis of the mechanism of the non-specific response phase may ultimately allow genetic engineering of broad-spectrum virus resistance in crop plants.