The acid tolerance response in Lactobacillus acidophilus CRL 639, induced at pH 4.2 for 15 min, is mediated by the cell membrane F-1-F-0 ATPase. The specific activity of the enzyme was induced 1.6-fold after acid adaptation compared to a non-adapted control. The ATPase was optimal at pH 6 with a K-m=0.8 mM and a V-max=100 mM.