Propionibacterium acnes CN-8, isolated from human dental plaque, was grown in a liquid medium, and its bacteriocin-like substance (acnecin) was extracted from the cells by ultrasonic treatment. Acnecin was purified to a homogeneous state with recovery of 47%. Specific activity increased 72-fold in comparison with the crude extract. The properties of acnecin were as follows. (i) Acnecin may consist of five subunits with a molecular weight of about 12,000. (ii) Its isoelectric point was 5.5. (iii) In amino acid composition, aspartic acid, glutamic acid, glycine, and alanine were predominant, whereas cystine was not present. (iv) Acnecin contained 3.3% carbohydrate but was substantially free from lipid. (v) The activity was lost by heating at 60 degrees C or by protease and lysozyme treatments. Acnecin acted bacteriostatically on the indicator strain without killing it. The action spectrum of acnecin was very narrow; it was effective only against strains of non-acnecin-producing P. acnes and Corynebacterium parvum, a species closely related to P. acnes.