SUMMARY The bacteriocin produced by Proteus morganii strain MR 336 is a thermo- labile glycoprotein with a sedimentation constant of 40. Activity is depen- dent on the integrity of the complex and the bacteriocin is inactivated by oxidation. Bacteriocinogeny could not be transmitted to other strains of P. morgunii but was eliminated by 30,ug./ml. acridine orange at pH 7.45. Bacteriocin-resistant mutants of a sensitive P. morganii strain were readily obtained.