The purpose of this study was to select and characterize a probiotic bacterium with distinctive antimicrobial activities. In this respect, Lactococcus lactis subspecies lactis Z11 (L. lactis Z11) isolated from Zabady (Arabian yoghurt) inhibited other strains of lactic acid bacteria and some food-born pathogens including Listeria monocytogenes, Bacillus cereus and staphylococcus aureus. The inhibitory activity of cell free supernatant (CFS) of L. lactis Z11 isolated from zabady was lost by proteolytic enzymes, heat resistant. Consequently, the active substance(s) of CFS was characterized as a bacteriocin. This bacteriocin has been shown to consist of protein but has no lipidic or glucidic moieties in its active molecule. Its activity was stable in the pH range 2.0 to 7.0 and was not affected by organic solvents. The L. lactis Z11 bacteriocin was produced in CFS throughout the mide to the late exponential phase of growth of the producer organism and maximum bacteriocin production was obtained at initial pH 6.5 at incubation temperature of about 30°C.