Resistance to plum pox virus (PPV) infection can be obtained in transgenic plants that express the virus capsid gene. An Agrobacterium-mediated transformation was used to introduce the PPV capsid gene into Prunus domestica plants. Over 11 regenerated plants (clones) were observed for the development of the disease symptoms and analysed for the presence of PPV by enzyme-linked immunosorbent assay (ELISA), Western blot analysis, and reverse transcription-polymerase chain reaction (RT-PCR) through 4 dormancy cycles. The level of protection against PPV was determined in the transformed plants, non-transformed plants, and a control transgenic plant "transformed" with the plasmid vector alone. One clone, C-5, appeared fully protected, while PT-6 and C-4 clones accumulated a low concentration of virus and the rest of the clones was entirely susceptible. Little is known about the mechanisms of resistance to virus infection in transgenic woody plants. To investigate this aspect, comparative studies based on the characteristics of resistant and susceptible clones have been started. A question, whether the phenotype resistance of clone C-5 is similar to that observed in transgenic herbaceous plants or not, has been addressed. Recent progress in this investigation is presented.