A full-length cDNA copy of potato virus A RNA was constructed downstream from the bacteriophage T7 RNA polymerase promoter. A single extra guanosine not present in PVA RNA was added to the 5'-end of the cDNA. The capped in vitro transcripts from cloned cDNA were infectious in the mesophyll protoplasts and intact plants of Nicotiana tabacum. PVA coat protein accumulated in transcript-inoculated tobacco protoplasts and infected systematically intact plants producing high PVA titres according to the ELISA and immunoblot assay. The infected tobacco plants produced similar mild mosaic symptoms in the systematically infected leaves, irrespective of whether the RNA transcripts or parental virus were used as inoculum; the virus particles were also morphologically similar according to immunosorbent electron microscopy.