Mesenterocin 52 (Mes-52), was produced by Leuconostoc mesenteroides ssp. mesenteroides strain FR52 and dextranicin 24 (per-24) by Leuconostoc mesenteroides ssp. dextranicum strain J24, Dex-24 had a very narrow spectrum of antibacterial activity: it inhibited only some other strains of Leuconostoc, contrary to Mes-52, which antagonized also Listeria and Enterococcus sp. Mes-52 was partially purified by ammonium sulfate precipitation, gel filtration, cation exchange, and hydrophobic interaction chromatography. Mes-52 was biosynthesized and excreted into the medium during growth phase. At different temperatures media, there was an inverse relationship between the final bacteriocin-specific productivity and the mean growth rate. After novobiocin treatment, different mutants of strain FR 52 were obtained: nonproducing-sensitive (Bac(-)Imm(-)) and nonproducing-immune (Bac(-)Imm(+)) strains. Many changes in the plasmid profiles of several mutants were detected by electrophoresis.