A bacteriocin, Fermentcin B produced by Lactobacillus fermentum, was identified from the inhibitory products of twenty-nine mesophilic lactic acid bacteria. It has a bactericidal activity with a narrow inhibitory spectrum. The bacteriocin is heat stable at 100 degrees C for 30 min and stable in pH range of 3.0 to 8.0. Fermentcin B lost its activity after treatment with alpha-chymotrypsin, proteinase-K, and amyloglucosidase.