Transgenic plants producing the 30K temperaturesensitive transport protein (TP) of tobacco mosaic virus (TMV) mutant Ni2519 (affecting cell-to-cell transport) were found to: (i) be susceptible to wild-type TMV U1 at 24 °C (a permissive temperature for Ni2519 TP), (ii) acquire a certain level of resistance to TMV U1 accumulation when maintained at 33 °C (a non-permissive temperature for Ni2519 TP) and (iii) lose the resistance to wild-type TMV after their transfer from 33 °C to 24 °C. It is suggested that reversible temperature-dependent conformational changes in Ni2519 TP are responsible for these phenomena and that production of a TP which is only partially functional in transgenic plants confers on these plants a resistance to the virus owing to reduction of the level of cell-to-cell transport. Transgenic tobacco plants producing the 32K TP ofbrome mosaic virus (BMV) acquired resistance to TMV U 1 suggesting that BMV TP is partially functional in tobacco plants.