Purified enterocin E1A, a bacteriocin produced by Streptococcus faecium E1, had a strong bactericidal activity on susceptible cells of Listeria monocytogenes. Electronmicroscopic observation of thin sections of L. monocytogenes, treated with enterocin E1A at a concentration that gave 0.1% survivors, revealed that the predominant features of morphological alterations concern the partial dissolution of the cell content and the damage of the cell envelope which enabled an efflux of cell-material. Not frequently intact protoplasts and greatly elongated cells were observed.