Lactococcus lactis QU 5 isolated from corn produces a novel bacteriocin, termed lacticin Q. By acetone precipitation, cation-exchange chromatography, and reverse-phase high-performance liquid chromatography, lacticin Q was purified from the culture supernatant of this organism, and its molecular mass was determined to be 5,926.50 Da by mass spectrometry. Subsequent analyses of amino acid and DNA sequences revealed that lacticin Q comprised 53 amino acid residues and that its N-terminal methionine residue was formylated. In contrast to most bacteriocins produced by gram-positive bacteria, lacticin Q had no N-terminal extensions such as leader or signal sequences. It showed 66% and 48% identity to AucA, a hypothetical protein from Corynebacterium jeikeium plasmid pA501, and aureocin A53, a bacteriocin from Staphylococcus aureus A53, respectively. The characteristics of lacticin Q were determined and compared to those of nisin A. Similar to nisin A, lacticin Q exhibited antibacterial activity against various gram-positive bacteria. Lacticin Q was very stable against heat treatment and changes in pH; in particular, it was stable at alkaline pH values, while nisin A was inactivated. Moreover, lacticin Q induced ATP efflux from a Listeria sp. strain in a shorter time and at a lower concentration than nisin A, indicating that the former affected indicator cells in a different manner from that of the latter. The results described here clarified the fact that lacticin Q belongs to a new family of class II bacteriocins and that it can be employed as an alternative to or in combination with nisin A.