Eleven strains of Lactococcus lactis synthesising substances of the properties antagonistic toward the strains of the same species were examined. The effect of the strains on pathogenic bacteria: Escherichia coli, Salmonella sp., Staphylcoccus aureus and Listeria monocytogenes was assessed by spot-diffusion method and on a basis of synthesis of inhibitory substance in a liquid medium. Seven out of eleven strains were found to inhibit the growth of the pathogenic bacteria. In the liquid medium only one strain, however, produced the inhibitory substance in the concentration required to inhibit the growth of L. monocytogenes and S. aureus, and, in the same conditions, three strains synthesised the substance of activity against only L. monocytogenes. These substances showed the character of bacteriocins since they did not inhibit the growth of gram-negative bacteria, lost the activity after incubation with proteolytic enzymes, were considerably thermostable in acid and neutral medium, and were inactivated after heating in basic medium. The antibacterial activity of bacteriocins found in this work limits their potential use for preventing the growth of pathogenic L. monocytogens and S. aureus.