Mutacin II is a lantibiotic that is produced by group II Streptococcus mutans. It inhibits the growth of other streptococci as well as many other gram-positive microorganisms by a hitherto unknown mechanism. Mutacin 11 possesses bactericidal activity against susceptible cells. It transiently depolarizes the transmembrane electrical potential (Δψ) and the transmembrane pH gradient (ΔpH) and partially inhibits amino acid transport. However, it rapidly depletes the intracellular ATP pool in glucose-energized cells and prevents the generation of ATP. It is concluded that mutacin II does not belong to the group of pore-forming lantibiotics (type A) or to the type B lantibiotics, which inhibit phospholipases or interfere with peptidoglycan biosynthesis. Mutacin 11 acts by inhibiting essential enzyme functions at the level of metabolic energy generation, an activity that has nut yet been classified for lantibiotics.