Leuconostoc spp. are lactic acid bacteria that are commonly associated with foods and that are used as starter bacteria in some dairy fermentations. Lactic acid bacteria are inhibitory to other bacteria because of pH, organic acids, hydrogen peroxide, and other chemicals produced during their growth, including bacteriocins. Bacteriocin production by Leuconostoc spp. was first observed in the 1950s, but only since 1984, when antagonistic activity of Leuconostoc spp. was reported, have more extensive studies of bacteriocins produced by Leuconostoc spp. been conducted, including mesentericin Y105, produced by Leuconostoc mesenteroides spp. mesenteroides; leucocin A-UAL 187, produced by Leuconostoc gelidum; carnosin 44A, produced by Leuconostoc carnosum; and leuconocin S, produced by Leuconostoc paramesenteroides. Bacteriocins produced by leuconostocs may or may not be active against other lactic acid bacteria, but all include Listeria in their activity spectra. Mesentericin Y105 is reported to be exclusively active against Listeria spp. The amino acid sequences for leucocin A and mesentericin Y105 have been determined. Despite considerable differences in antibacterial spectra, only two amino acids differ between these bacteriocins. The prevalence of leuconostocs in many adventitious fermentations of food and the use of leuconostocs as starter bacteria in controlled fermentations make the bacteriocins produced by these bacteria of interest as possible food preservatives by addition of the bacteriocin or its producer organism to foods.