Log-phase Listeria monocytogenes cells become tolerant to a variety of environmental stresses following acid adaptation at pH 5.5. We demonstrated that adapted cells also exhibit increased tolerance to nisin and, to a lesser extent, lacticin 3147. At nisin concentrations of 100 and 200 IU/ml the survival of acid-adapted cells was approximately 10-fold greater than nonadapted cells. However, acid adaptation had only a moderate effect on the tolerance of L. monocytogenes to lacticin 3147, a phenomenon that possibly reflects the distinct mode of action of this bacteriocin. Analysis of the fatty acid composition of the bacterial membrane indicated that straight-chain fatty acids C14:0 and C16:0 were significantly increased in acid-adapted cells while levels of C18:0 decreased. The results indicate that stress mechanisms that are induced in mildly acidic conditions provide protection against the antimicrobial action of bacteriocins. This increased resistance of acid-adapted L. monocytogenes could cause increased survival of this pathogen in food products in which nisin or other bacteriocins are used as preservatives.