The peptide antibiotic nisin is shown to disrupt valinomycin-induced potassium diffusion potentials imposed on intact cells of Staphylococcus cohnii 22. Membrane depolarization occurred rapidly at high diffusion potentials while at low potentials nisin-induced depolarization was slower suggesting that nisin requires a membrane potential for activity. This assumption was proven in experiments with planar lipid bilayers (black lipid membranes). Macroscopic conductivity measurements indicated a voltage-dependent action of nisin. The potential must have a trans-negative orientation with respect to the addition of nisin (added to the cis-side) and a sufficient magnitude (ca.-100 mV). With intact cells the threshold potential was lower (-50 to -80 mV at pH 7.5 and below -50 mV at pH 5.5). Single channel recordings resolved transient multi-state pores, strongly resembling those introduced by melittin into artificial bilayers. The pores had diameters in the range of 0.2-1 nm, and lifetimes of few to several hundred milliseconds. The results indicate that nisin has to be regarded as a membrane-depolarizing agent which acts in a voltage-dependent fashion.