There are several strains of Enterococcus mundtii that produce a class IIa bacteriocin, mundticin, along with a cognitive immunity protein and ABC transporter. However, E. mundtii CUGF08 and ATO6 are not immune to their own bacteriocin production, like the other bacteriocinogenic strains. Since it was found that the presence of proteinase K inactivated the self-lethality phenotype, peptides produced by E. mundtii CUGF08 were isolated from its supernatant though chromatography (solid-phase extraction, cation-exchange, and RP-HPLC). The intact mass of one causative agent was 6 kDa and its trypsin-digested fragment sequence was determined to be AIGIIGNNSAANLATGGAAGWK. The fragment is 100% identical to the C-terminal sequence of mundticin L, but the intact mass corresponds more closely to the precursor peptide. Conversely, SDS-PAGE analysis displayed only a 4-kDa peptide from the supernatant that showed activity to E. mundtii CUGF08. After Edman degradation was performed on the band, the N-terminal sequence was found to be KYYGNGLSXNKKGXSVDX(G)(K)A(I)(G)(I), which matches with mundticin L. Since there are several strains that react differently to mundticin homologs, the genomes of four mundticin producers and one non-mundticin producer were compared in order to identify potential genes involved with mundticin immunity or target specificity. The two aforementioned strains, along with non-producer strain E. mundtii ATCC 882, are sensitive to mundticin,. Two producers in literature, E. mundtii QU 25 and CRL35, were reported to be immune to their purified mundticin, There are 28 genes unique to the immune strains and 3 genes unique to the sensitive strains. Many of those found in the immune strains are phage proteins. One unique gene from the immune strains may be a secreted protein or a lipoprotein, which could be involved with the additional immunity. One unique gene from both sensitive strains shows homology to a transmembrane protein, which may be an additional cell membrane receptor for mundticin. Further work with complementation studies with those genes may determine additional, uncharacterized mechanisms of immunity and action, especially since many of those mechanisms are poorly understood for class IIa bacteriocins.