Abstract: L. casei RN 78 isolated from a dairy sample was shown to produce a proteinaceous substance inhibitory towards a number of gram positive bacteria including an important food-borne pathogen L. monocytogenes. The inhibitory substance produced by this strain appeared not related to organic acids and hydrogen peroxide and was found resistant to catalase, lipase and lysozyme. The proteinaceous antagonistic nature of the agent appeared to be related to a bacteriocin and was termed Lactocin RN 78. The stability of the crude Lactocin RN 78 in the presence of different physical and chemical factors was evaluated which indicated its stability at wide temperature and pH range. The spectrum of inhibition of this bacteriocin was further enhanced to the growth of E. coli, Ps. aeruginosa, and S. typhi in the presence of 1% chelating agents like SDS and EDTA. Lactocin RN 78 appeared stable in the presence of high concentration of NaCl and resisted 40% of this salt during 48 h of incubation at room temperature. The kinetics of Lactocin RN 78 production by L. acidophilus RN 78 as a function of cellular condition was evaluated, which indicated a direct relationship between the growth rate (cfu mL-1) and the amount of bacteriocin produced. The synthesis and inhibitory activity started in the early logarithmic phase and continued to the end of exponential phase. The growth became static after entering the stationary phase within 10 h of incubation at 35°C and the antibacterial titer remained constant thereafter. The influence of variable physical and chemical agents on the growth metabolism and Lactocin RN 78 synthesis together was investigated. The optimal temperature and pH favoring maximum growth and production in the producer strain was 35°C and 6.0, respectively. The producer strain was able to grow and produce Lactocin RN 78 in the presence of 3% NaCl and 0.3% bile salts. The growth medium most effective was MRS broth medium supplemented with a carbon source.