The extracts of 33 plants were evaluated for their effects on growth of Escherichia coli O157:H7 (EHEC). The extract of Haematoxylon brasiletto was the only one that effectively inhibited bacterial growth. The effects of ethanolic extracts of this plant on growth, verotoxin production, and adhesion of E. coli O157:H7 to HeLa cells were determined. The MBC for growth was 4 mg/ml. No verotoxin formation was detected at 1, 2, or 3 mg/ml. Preexposing bacteria and HeLa cells to various concentrations of extracts affected the adhesion between non-EHEC and HeLa cells. Partial purification of the active fraction suggested that polyphenols might play a role in the antimicrobial activity exhibited by H. brasiletto extracts.