SUMMARY The properties of a bacteriocin derived from Lactobacillus fermenti strain 466 were investigated. The bacteriocin was present in low titre in supernatant fluids from overnight broth cultures and was not inducible by ultraviolet radiation. It was purified and concentrated to a titre of I/IOOO by dialysis, chromatography on Sephadex G IOO and calcium phosphate gel columns. The bacteriocin is heat stable, and sensitive to trypsin and pepsin but not to lysozyme. No migration was demonstrated in electrophoretic fields in agar gel. Electron microscopy of the bacteriocin did not show any phage components. The bacteriocin is a macromolecular lipocarbohydrate protein which consists of 16 amino acids, four sugars, hexosamine and phosphorus. The biological activity of this complex is dependent on its structural integrity.