Bacteriocin-like activity (BLA) was screened in 690 strains of lactic acid bacteria isolated from plant materials such as silages and fermented vegetables. Among them, a strain identified as Enterococcus faecium NIAI 157 showed a clear BLA against the indicator strain, Ent. faecium IFO 13712. The proteinaceous nature and antimicrobial activity against closely related species strongly indicated that this BLA was a bacteriocin and was designated enterocin ON-157. The bacteriocin activity of this strain was extracellularly produced in the logarithmic growth phase in MRS broth and purified by ultrafiltration, ammonium sulphate precipitation and cation-exchange chromatography. Purified enterocin ON-157 had a molecular weight of approximately 2500 Da in SDS-PAGE analysis and was easily inhibited by treatment with alpha-amylase and proteolytic enzymes. Enterocin ON-157 had a bactericidal mode of action and inhibited the growth of the enterococci, Lactobacillus sake and Listeria monocytogenes. Enterococcus faecium NIAI 157 harboured two plasmids, 49.0 kb and 43.7 kb, and a variant missing a larger plasmid by curing with novobiocin lost the bactriocin activity.