Technical Abstract: Pathogenic bacteria internalized in leaf tissues are not effectively removed by surface treatments. Ionizing radiation has been shown to inactivate leaf-internalized bacteria, but many aspects of targeting these protected pathogens remain unknown. Leaves of Iceberg, Boston, green leaf and red leaf lettuce were cut into pieces, submerged in a cocktail mixture of three isolates of Escherichia coli O157:H7 and subjected to a vacuum perfusion process to force internalize the bacterial cells into the intercellular spaces of the leaves. The inoculated leaves were then treated with a 3 min water wash, a 3 min wash with a sodium hypochlorite sanitizing solution (300 ppm or 600 ppm), or various doses of ionizing radiation (0.25 - 1.5 kGy). Leaves were stomached to recover the internalized cells and survivors enumerated. Water, 300 ppm and 600 ppm washes each gave reductions of less than 1 log; these were statistically significant only in the case of green leaf lettuce. In contrast, all doses of irradiation significantly reduced E. coli O157:H7 on all lettuce varieties examined. The specific variety influenced the efficacy of irradiation. The greatest reduction obtained was 5 logs on Iceberg lettuce, treated with 1.5 kGy. The D10 values (the dose necessary to achieve a 1 log reduction) were significantly (P<.05) different among the varieties of lettuce tested, and ranged from 0.30 kGy (Iceberg) to 0.45 kGy (Boston). These values were observed to be notably higher than previously published irradiation D10 values for E. coli O157:H7 surface inoculated onto these four lettuce varieties. This study has shown that irradiation is able to effectively reduce viable E. coli O157:H7 cells internalized in lettuce, and that the variety of lettuce influences the specific response.