The relationship between titrated acidity and pH was studied using the 3206 samples of human gastric juice, which were collected by the intragastric tube in the basal and gastrin stimulated conditions. The pH of samples subjected to the present study ranged from 0.95 and 6.9. The results showed that a certain value of pH did not always indicate an identical value of titrated acidity, and that the coefficient of variation was greater as pH value rose. As a whole, however, the acidity became lower with the increase of pH value. Thus, the relationship between pH and titrated acidity could be expressed closely as "Y = 369.19 - 424.09X + 203.66X2 - 48.29X3 + 5.57X4 - 0.25X5), where X = pH and Y = acidity. The conversion table from pH to acidity, which was calculated by this expression, was presented for the clinical use.