To study the effect of cell-to-cell movement of potato virus X (PVX) on the size-exclusion limit of plasmodesmata we have microinjected virus particles and fluorescently labelled dextrans (F-dextrans) with average molecular mass of 4.4, 10, and 20 kDa into Nicotiana clevelandii trichome cells. A vector construct of PVX expressing GUS (PVX.GUS) moved from the injected cell and modified the plasmodesmata to allow passage of the 4.4- and 10-kDa F-dextrans but not the 20-kDa F-dextran. A mutant PVX with a deletion in the gene for the 25-kDa protein accumulated in inoculated protoplasts to the same level as the wild-type PVX. This mutant virus did not move from the injected cell and did not modify the plasmodesmata to allow passage of the 10-kDa F-dextran. However, the 4.4-kDa F-dextran moved from the cells infected with the mutant virus. These data therefore illustrate that movement of PVX is associated with modification of plasmodesmata. The full plasmodesmatal modification requires a functional 25-kDa protein but partial modification occurs when this protein is present in a mutant form.