A novel antibacterial substance produced by a strain isolated from Bulgarian yellow cheese was characterized. The producer strain was identified by molecular typing to belong to the species Lactobacillus delbrueckii, which is a rare producer of bacteriocins. The inhibitory agent was heat stable and active against lactic acid bacteria species and several food-borne pathogens: Listeria monocytogenes, Staphylococcus aureus, Enterococcus faecalis, Escherichia coli, Yersinia enterocolitica and Y. pseudotuberculosis. Its sensitivity to amylolitic enzymes and lipase suggested that a lipid and carbohydrate moiety could be important for the activity. The amino acid content of the purified bacteriocin was estimated to 29 amino acids. The bacteriocin was shown to be small (3.6-6 kDa) by three different methods: HPLC gel-filtration, SDS-PAGE and amino acid contents.