Five transgenic clones of Prunus domestica L. containing plum pox virus (PPV) coat protein (CP) gene and one non-transformed control clone were challenged with PPV-S in the field. Symptoms developed on C2, C3, C4, C6 and B70146 but not C5 trees inoculated by chip budding (CBI) (2/2, 2/2, 1/1, 2/2 and 2/2, positive/inoculated) in the first summer after inoculation. However, in the second year, symptoms appeared on CBI C5 trees. The presence of the virus in the plants was confirmed by enzyme-linked immunosorbent assay (ELISA) and reverse transcription-polymerase chain reaction (RT-PCR) amplification of a fragment of viral polymerase gene. During two years, symptoms of infection developed on 3 to 4 of 8 non-inoculated trees of clones C2, C3, C4, C6 and B70146. Eight non-inoculated C5 trees remained symptomless and ELISA-negative as of spring 1998, in spite of the presence of aphid vectors and inoculum sources.