Full-length cDNA clones of potato virus X (PVX) strains PVXUK3 and PVXHB have been constructed in plasmid vectors to allow in vitro transcription of infectious PVX RNA. In both instances the transcript-derived virus infected tobacco and potato identically to the respective progenitor strains: in tobacco and susceptible potato cultivars both strains infected systemically, producing symptomless or mild mosaic symptoms. In potato carrying the Rx or Nx resistance genes, the virus derived from the PVXHB cDNA infected systemically, whereas the virus derived from the PVXUK3 cDNA failed to infect the Rx plants or induced apical necrosis, characteristic of a hypersensitive response of the Nx gene. Three hybrid viral genomes were constructed at the cDNA level to localize the resistance breaking determinants of PVXHB. Transcripts of all three hybrids were infectious on tobacco. On potato cultivars with either the Rx or Nx resistance genes, the hybrid viruses infected in the same way as PVXHB, rather than PVXUK3. The common feature of these hybrid viruses, the coat protein gene, is therefore the determinant of Nx and Rx resistance breaking of PVXHB.