1. The most powerful adsorbents of pepsin are aluminum hydroxide gel and charcoal. 2. The adsorbent action of pepsin is uninfluenced by the substrate concentration and is less active at lowpH levels. When used to inactivate pepsin, the adsorbent effect of aluminum hydroxide gel is more important than thepH effect. 3. The capacity of aluminum hydroxide gel to adsorb other substances such as anticholinergic agents and antibiotics should be remembered when it is used in the treatment of peptic ulcer. 4. The antipeptic capacity of aluminum hydroxide gel and the secretory responses found in ulcer patients have been discussed in relationship to the dosage used in ulcer therapy.