Enterococcus faecium 108 was isolated from food by the FDA³s Listeria selective enrichment procedure. E. faecium 108 produces an extracellular anti-listerial compound, enterocin B108. The anti-listerial compound is a protein being inactivated by treatment with proteinase K, trypsin and chymotrypsin. It is active against all Listeria species, heat stable (90°C for 30 min) and is very stable during storage at -70° to 5°C. Enterocin B108 is rapidly bacteriocidal without cytolysis. However, eventually membrane damage becomes evident in electron micrographs. The approximate molecular weight (M.W.) was 10,000 by SDS-PAGE. Enterocin B108 was purified to electrophoretic homogeneity by gel-filtration, ion exchange and reverse phase chromatography using the Fast Protein Liquid Chromatography system, although the yield was insufficient to determine the sequence. Poor yield was due to interference by a pigment-like compound in the growth medium. Anti-listerial activity that is free of pigment was obtained in high yield by adjustment of washed cells to pH 2 with phosphoric acid (5% v/v). A subsequent one-step purification by gel-filtration in 5M urea produced a homogeneous protein of M.W. 10,000.