A cDNA clone of the gene encoding attacin was used to construct three plasmid binary vectors in which attE was controlled by the cauliflower mosaic virus 35S promoter with duplicated upstream B domain (35S) (p35SAtt), 35S with the untranslated leader sequence of alfalfa mosaic virus RNA 4 (AMV) (p35SAMVAtt), and 35S with AMV and the signal peptide of pathogenesis-related protein 1b from tobacco (SP) (p35SAMVSPAtt), respectively. These plasmids and pLDB15 containing attE under the control of the potato proteinase inhibitor II (Pin2) promoter were used in Agrobacterium-mediated transformation of the apple scion cultivar `Galaxy＇ and the apple rootstock M.26 to enhance resistance to Erwinia amylovora, the bacterium that causes fire blight. The mean attacin content of transgenic lines containing attacin with AMV was three times higher than lines without AMV. Northern blots suggested that AMV functioned in apple as it does in other plant species by enhancing translation of attE mRNA. Transgenic ＇Galaxy＇ lines with attacin fused to SP had lower attacin content than lines without SP. In vitro assays indicated that attacin was partially degraded in the intercellular fluid of apple leaves. However, transgenic `Galaxy＇ lines transformed with attacin fused to SP had significantly less disease than those without SP suggesting that intercellularly secreted attacin is more effective in reducing E. amylovora infection than intracellularly localized attacin. A negative correlation was observed between attacin content and disease resistance in Pin2Att transgenic ＇Galaxy＇ lines following inoculation with E. amylovora, suggesting that attacin enhances resistance to fire blight.