Klebsiella pneumoniae NCTC 418 was cultured aerobically in chemostat cultures (D = 0.3 h-1; 35 degrees C) under respectively carbon-, phosphate-, potassium-, sulphate-, and ammonia-limited conditions with glucose as the sole carbon and energy source. The effect of the external pH value on glucose metabolism and on the enzymes of the direct glucose oxidative pathway was examined. The pH value of the medium had a profound influence on both the activity and the synthesis of the glucose dehydrogenase and the gluconate dehydrogenase. At pH values ranging from pH 5.5 to pH 6.0 maximal activity and synthesis of these enzymes resulted in a more than 80% conversion of the glucose consumed into gluconate and 2-ketogluconate under potassium- or phosphate-limited conditions. On the other hand, no gluconate and/or 2-ketogluconate production could be detected when K. pneumoniae was cultured at pH 8.0. Whereas the synthesis of gluconate dehydrogenase seemingly was completely repressed, still some glucose dehydrogenase was present. The lack of glucose dehydrogenase activity at pH 8.0 was shown not to be due to the dissociation of the cofactor PQQ from the enzyme.