The mode of action of cerein 8A, a bacteriocin produced by the soil bacterium Bacillus cereus 8A, was investigated. The effect of cerein 8A was tested against Listeria monocytogenes and a bactericidal effect at 400 arbitrary units (AU)/ml was observed. In addition, cerein 8A was bactericidal against Bacillus cereus at 200 AU/ml, and inhibited the growth of Escherichia coli and Salmonella Enteritidis. Stronger inhibition of these gram-negative bacteria was achieved when the chelating agent EDTA was added together with bacteriocin. The effect of cerein 8A on B. cereus and L. monocytogenes was also investigated by Fourier transform infrared spectroscopy (FTIR). Treated cells had an important frequency increase at 2920 cm^–1 and a decrease at 1400 cm^–1, corresponding to assignments of fatty acids. Transmission electron microscopy showed damaged cell walls and loss of protoplasmic material. These results suggest that the mode of action of cerein 8A is to interfere with cell membranes and the cell wall. [Int Microbiol 2005; 8(2):125-131]