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Clinical and food Listeria monocytogenesisolates, pre-exposed to mild acidic conditions, were able to readily develop acid tolerance, irrespective of their origin. We attempted to investigate the influence of acid tolerance mechanisms, either constitutive or induced, on the invasive behaviour of this facultative food-borne pathogen. Entry efficiency and intracellular growth of acid-tolerant strains were evaluated in in vitrocell models capable to mimic in vivotarget cells, such as enterocytes and macrophages. An acid-adapted L. monocytogeneswild-type strain and a constitutively acid-tolerant mutant were able to enter enterocyte-like (Caco-2) cells as well as to survive and proliferate intracellularly in lipopolysaccharide-treated macrophage-like (J774.A1) cells, at a significant increased extent by respect of the non acid-adapted wild-type strain. These findings add new information about the influence of the acid tolerance response on L. monocytogenesvirulence, suggesting that in acid-adapted bacteria the early events of pathogenesis which allow the colonization and the spread of bacteria in the host may be highly promoted.