In connection with the testing of an antacid in form of a suspension, pH-measurement and gastroscopy are carried out in the same 11 patients. Essential criteria of the efficacy of an antacid are its buffering capacity and its retention in the stomach as a mechanical protective for the gastric mucosa. The Talcid suspension tested in this way showed a good period of stay in the stomach, often exceeding the period of acid-binding capacity. Compared with bicarbonate, its buffering period is about twice as long above pH 4.5 and about 2 1/2 times as long at lower pH's.