Pepstatin is shown to be a specific inhibitor of acid proteases. It also inhibits humangastricsin, but the effect is weaker than against humanpepsin. The content of pepsin and gastricsin in gastric juice of stomach ulcer patients is described and the pepstatin which remains in gastric juice collected at 60 minutes after its administration is also determined. These results and those obtained by the direct measurements of the peptic activity of the gastric juice indicate that a sufficient amount of pepstatin to inhibit pepsin remains in the