Listeria monocytogenes acquired increased acid tolerance during exponential growth upon exposure to sublethal acid stress, a response designated the acid tolerance response (ATR). Maximal acid resistance was seen when the organism was exposed to pH 5.0 for 1 h prior to challenge at pH 3.0, although intermediate levels of protection were afforded by exposure to pH values ranging from 4.0 to 6.0. A 60 min adaptive period was required for the development of maximal acid tolerance; during this period the level of acid tolerance increased gradually. Full expression of the ATR required de novo protein synthesis; chloramphenicol, a protein synthesis inhibitor, prevented full induction of acid tolerance. Analysis of protein expression during the adaptive period by two-dimensional gel electrophoresis revealed a change in the expression of at least 23 proteins compared to the non-adapted culture. Eleven proteins showed induced expression while 12 were repressed, implying that the ATR is a complex response involving a modulation in the expression of a large number of genes. In addition to the exponential phase ATR, L.monocytogenes also developed increased acid resistance upon entry into the stationary phase; this response appeared to be independent of the pH-dependent ATR seen during exponential growth.