Cells of a strain of Lactobacillus lactis selected for ability to produce hydrogen peroxide were added to Trypticase soy broth (TSB) containing Escherichia coli O157:H7 to determine if L. lactis was antagonistic toward the E. coli during storage at 7°C for 7 days. E. coli was enumerated on violet red bile agar. Three strains of E. coli O157:H7 (43894, 43890, and 35150) were evaluated. Control samples containing no L. lactis did not show significant declines in numbers of E. coli during the 7 days of storage. However, samples inoculated with at least 5.0 x 107 L. lactis per ml exhibited significant declines in numbers of E. coli after only 3 days of storage for all strains. Samples inoculated with fewer L. lactis displayed varying effects on E. coli O157:H7 depending on the strain. E. coli O157:H7 strain 43894 appeared to be the most resistant to the antagonistic action of the L. lactis. Interaction experiments in the presence of catalase indicated that hydrogen peroxide was the main factor responsible for the inhibitory action produced by the lactobacilli. Raw chicken breast meat inoculated with E. coli O157:H7 strain 43894 plus the cells of L. lactis and stored at 5°C exhibited declines in numbers of the pathogen, whereas those inoculated only with the E. coli exhibited no declines during storage at 5°C.