Lactobacillus plantarum ST31 isolated from sourdough produced an antimicrobial substance inhibiting other strains of the genera Lactobacillus, Leuconostoc, Pediococcus, Streptococcus, Bacillus and some foodborne pathogens including Staphylococcus aureus. This antimicrobial substance was inactivated by proteolytic enzymes. Consequently, it was characterized as a bacteriocin and was designated plantaricin ST31. This bacteriocin was stable in the pH range 3-8 and it was not affected by amylolytic enzymes. Production of plantaricin was pH and temperature dependent, and maximum yields were obtained in MRS broth cultures maintained at pH 6 and incubated at 30 degrees C in the exponential phase to the early stationary growth phase of the producer organism. This bacteriocin was purified by using consecutive ammonium sulfate and reversed-phase chromatography. It is a peptide of 20 amino acid residues with a mass of 2755+/-0.3 Da, as determined by electrospray mass spectrometry. The sequence of Plantaricin ST31 showed no similarity to those of other bacteriocins. Plantaricin ST31 production appeared to be chromosomally encoded.