Cerein 7 is a peptidic antibiotic produced by Bacillus cereus Bc7 (CECT 5148) at the end of exponential growth but before sporulation onset. Cerein 7 has a broad spectrum of antibacterial activity against Gram-positive bacteria, but it is inactive against Gram-negative bacteria. The sequence of its amino-terminal end and its characteristics of hydrophobicity and molecular mass make cerein 7 unique among the bacteriocins produced by the soil bacterium B. cereus. In this paper a further characterization of cerein 7 is presented, it is shown that it can be classified as a Klaenhammer's class II bacteriocin and that its mode of action corresponds to that of a membrane-active compound.