Monomolecular layers of lipids at the air/water interface have been used as a model membrane to study membrane interactions of the lantibiotic nisin. The natural lantibiotics nisin A and nisin Z proved to have a high affinity for the anionic lipids phosphatidylglycerol and bis(phosphatidyl)glycerol (cardiolipin). The interaction with zwitterionic phopholipids or neutral lipids is very low at surface pressures higher than 32 mN/m. Nisin, nisin mutants and lacticin 481 show a remarkable correlation between anti-microbial activity and anionic lipid interaction. The results indicate that primarily the N-terminal part (residues 1-22) penetrates into the lipid phase. Reduction of the flexibility at positions 20-21 has a negative effect on monolayer interaction and activity. The C-terminal part is probably responsible for ionic interactions of nisin in monomeric or oligomeric form with anionic lipids. In mixtures of anionic and zwitterionic lipids maximal interactions are found at approximately 70 mol/100 mol anionic lipid. Gram-positive bacteria, which form the main target for nisin, are characterized by a high content of anionic lipids in the membrane. Monolayers formed of lipid extracts of bacteria sensitive to nisin were more strongly penetrated than those of bacteria relatively insensitive to nisin.