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Streptococcus thermophilus 19258 and 573, Luctococcus luctis ssp. lucris C2, Lactococcus luctis ssp. cremoris HP, and Luctobucillus cusei 685 were grown in various media and assayed for protontranslocating ATPase (H+-ATPase) activity. The H+-ATPase was extracted from the membrane fractions of protoplasted cells using a French pressure cell, and activity was measured by the release of inorganic phosphate from ATP. The pH optima for the H+-ATPase, assayed in vitro, were 7.5 for S. thermophilus and Lactococcus luctis ssp. cremoris HP and 5.0 for Lactobacillus casei 685. In contrast, for cells grown in batch culture the H+-ATPase activity was always greatest when the cytoplasmic pH was less than the optimum for the enzyme. The H+- ATPase activity generally increased as the pH decreased until an extracellular pH of 5.0 was reached. Below an extracellular pH of 5.0, activities of this enzyme dropped markedly. The aciduric lactic acid bacterium, Lactobacillus casei 685, had higher basal levels of this enzyme than the streptococci and lactococci. Results suggest that this enzyme may be involved in regulation of the intracellular pH in lactic acid bacteria.