The acid tolerance of Leuconostoc oenos was examined in cells surviving at pH 2.6, which is lower than the acid limit of growth (about pH 3.0). Acid-adapted cells survived better than non-adapted cells. Tolerance to acid stress was found to be dependent upon the adaptive pH. Acid resistance was increased by an order of magnitude for cultures adapted to a pH of about 2.9. Inhibiting protein synthesis with chloramphenicol prior to acid shock revealed that acid adaptation may involve two separate systems, one of which appears to be independent of protein synthesis. The acid-resistant mutant LoV8413, isolated during a longterm survival screen at pH 2.6, was found to be able to grow in acidic media and was characterized by a high H+-ATPase activity at low pH. The data from electrophoretic analysis of total proteins labeled with [S-35]methionine indicate that large amounts of a protein of 42 kDa molecular mass were produced within this acid-resistant mutant.