The acid response and the correlated protein synthesis in Listeria monocytogenes were studied. The lowest pH value which L. monocytogenes could resist was dependent on the strain and the kind of acid used. Previous adaptation to an intermediary pH augmented bacterial resistance to a subsequent lethal acidic pH. The acid tolerance was also growth phase dependent. Organic volatile acids exerted a more deleterious effect on L. monocytogenes than inorganic acids, because weak acids infer a lower intracytoplasmic pH. The responses to acid adaptation (mildly acidic pH) and acid stress (lethal acidic pH) shared the majority of acid-induced proteins. The bacteria required more stress proteins to face severe acidic conditions. In order to obtain insights into the role these acid-induced proteins play in the mechanism of acid resistance, the proteins were analysed by two-dimensional electrophoresis and the most abundant were identified by peptide mass fingerprinting and MALDI-TOF mass spectrometry.