Titration to different pH end points was performed in all samples of gastric juice from 208 patients with gastro- instestinal diseases and healthy persons, examined with the augmented histamine test. Titration to pH 3.3 and to pH 7.0 revealed differences in hydrogen ion concentration, which were significantly negatively correlated to the maximal acid output, calculated from the acidity at pH 3.3. Differences between maximal acid outputs, calculated from the acidities at pH 3.3 and pH 7.0, varied only little in all examined groups with the exception of gastric juice contaminated by food. Titration to pH 7.0 or higher overestimates gastric hydrochloric acid, especially in gastric juice from patients with gastritis and gastric juice contaminated by food. The results of this study support previous investigations, which recommend titration of gastric juice to pH 3.5 with simultaneous measurement of pH. Achlorhydria corresponding to absence of parietal cells is hereby defined by a pH higher than 3.5 and a pH fall less than one pH unit from basal to maximally stimulated values.