Forty-six strains of genus Lactobacillus from the collection of the National Bank for Industrial Microorganisms and Cell Cultures (NBIMCC) were screened by the use of the agar drop diffusion test, for antibacterial activity against nine test-microorganisms. Lactobacillus casei 300 exhibited activity against eight lactic acid bacteria and one yeast strains. The substance with antibacterial activity, produced by the NBIMCC 300 strain, was inactivated by proteolytic enzymes and possessed thermostability when heated to 80°C. This gave us a reason to conclude that this substance belonged to the bacteriocins' group.