The use of antimicrobial agents in the management of patients infected with Shiga toxinproducing Escherichia coli (STEC) O157 : H7 remains controversial. Here, we examined the antibacterial efficacy of a natural product, ellagitannin from Quercus infectoria (Qi 4), against the organism in a murine model. Streptomycin-pretreated mice were orogastrically inoculated with 2¾10 9 c.f.u. streptomycin-resistant E. coli O157 : H7. The results demonstrated a stable high level of STEC present in the faeces of the infected animals. The bacterial levels in infected mice receiving Qi 4 at MIC and 2¾MIC were not significantly different from those of the untreated group (t-test; P.0.05). In contrast, Qi 4 at 4¾MIC significantly reduced the numbers of STEC within 2 days (t-test; 0.05.P.0.01). No viable bacteria were detected between day 5 and day 10. Similarly, at day 10, no organisms were detected from the intestines of the Qi 4-treated group, while they were recovered at levels of 10 8-11 and 10 5-10 c.f.u. g "1 in the colons and caeca of the infected mice, respectively. Histopathological findings from the infected kidneys revealed a marked increase in the number of mesangial cells and mesangial matrix. Ultrastructural examination of the kidneys from the infected mice also demonstrated proliferation of mesangial cells and an increase in the mesangial matrix. Cellular injury of endothelial cells with irregular borders and cytoplasmic bleb formation were noted. In contrast, the effects were not observed in the animals treated with Qi 4. The results clearly indicated that administration of Qi 4 could effectively eradicate the colonization of STEC O157 : H7 in the intestinal tract of mice and prevent renal injury. This compound may be an alternative candidate for a therapeutic agent against infections caused by this dangerous organism.