A synthetic gene encoding a single chain Fv fragment of an antibody directed against the nuclear inclusion a (NIa) protein of potato virus Y (PVY) was used to transform two commerical potato cultivars (Claustar and BF15). The NIa protease forms the nuclear inclusion body A and acts as the major protease in the cleavage of the viral polyprotein into functional proteins. Immunoblot analysis showed that most of the resulting transgenic plants accumulate high levels of the transgenic protein. Furthermore, a majority of the selected transgenic lines showed an efficient and complete protection against the challenge virus after mechanical inoculation with PVY^o strain. Two transgenic lines showed an incomplete resistance with delayed appearance of symptoms accompanied by low virus titers, whereas one line developed symptoms during the first days after inoculation but recovered rapidly, leading to a low virus accumulation rate. These results confirm that expression of scFv antibody is able to inhibit a crucial step in the virus multiplication, such as polyprotein cleavage is a powerful strategy for engineered virus resistance. It can lead to a complete resistance that was not obtained previously by expression of scFv directed against the viral coat protein.