SUMMARY: A Lactobacillus plantarum bacteriocin, plantaricin A, has been purified to homogeneity by ammonium sulphate precipitation, binding to cation exchanger and Octyl-Sepharose, and reverse-phase chromatography. The bacteriocin activity was associated with two peptides, termed α and β, which were separated upon reverse-phase chromatography. Bacteriocin activity required the complementary action of both the α and β peptides. From the N-terminal end, 21 and 22 amino acid residues of α and β, respectively, were sequenced. Further attempts at sequencing revealed no additional amino acid residues, suggesting that either the C terminus had been reached or that modifications in the next amino acid residue blocked the sequencing reaction. Judging from their amino acid sequence, α and β may be encoded by the same gene, since α appeared to be a truncated form of β. Alanine, the first amino acid residue at the N-terminal end of β was not present at this position in α. Otherwise the sequences of α and β appeared to be identical. The calculated molecular masses of the sequenced part of α and β were 2426 and 2497 Da, respectively. The molecular masses of α and β as determined by mass spectroscopy were 2687 ± 30 and 2758 ± 30 Da, respectively, indicating that (i) the only difference between α and β was the presence of the N-terminal alanine residue in β, and that (ii) in addition to the sequenced residues, two to three unidentified amino acid residues are present at the C-terminal ends of the α and β peptides. Both α and β may form amphiphilic α-helices, suggesting that they are pore-forming peptides that create cell membrane channels through a ‘barrel-stave' mechanism.