Lactobacillus brevis SB27, isolated from sausages, produced an antimicrobial substance active against numerous strains of heterofermentative lactobacilli and against some strains of pediococci and Bacillus. The antibacterial agent was shown to be heat stable, resistant over a wide pH range, and sensitive to proteolytic enzymes. It was identified as a bacteriocin and termed brevicin 27. Dialysis and ultrafiltration suggested an apparent molecular weight between 10 and 30 kDa for the crude inhibitory molecule. Brevicin 27 exhibited a hydrophobic character. A partially purified preparation, resulting from ammonium sulfate precipitation and cation exchange chromatography, permitted confirmation of some characteristics of the bacteriocin, previously established with the crude extract. After treatment of the original brevicin 27-producing strain with novobiocin, a nonproducing mutant was obtained. This mutant was sensitive to brevicin 27, and its plasmid profile revealed the loss of a plasmid of about 3 MDa.