The antimicrobial activities of bovine lactoferrin (bLF), its pepsin hydrolysate (bLFH) and the active peptide lactoferricin B (LFcinB) against four clinical isolates of enterohaemorrhagic Escherichia coli O157:H7 were studied. The MICs against these isolates were 3 mg ml-1 for bLF, 0.1-0.2 mg ml-1 for bLFH and 8-10 micrograms ml-1 for LFcinB in 1% Bactopeptone broth. LFcinB killed these bacteria within 3 h at concentrations above 10 micrograms ml-1. Transmission electron microscopy findings suggested that LFcinB acts on the bacterial surface and affects cytoplasmic contents. LFcinB was shown to influence the levels of verotoxins in the culture supernatant fluid of an E. coli O157:H7 strain. These results demonstrate that E. coli O157:H7 strains are susceptible to the antimicrobial effects of bLF and its peptides.