1. The ability of homogenates of jejunal mucosa to liberate amino acids from a peptic-tryptic digest of gluten was assessed. Mucosal specimens were obtained from thirty-two control subjects without malabsorptive disease, from twenty-six patients with untreated adult coeliac disease and from nine patients with adult coeliac disease in whom the intestinal mucosa was histologically normal as a consequence of treatment with a gluten-free diet. In addition nine of the untreated patients were restudied after institution of a gluten-free diet. 2. The ability of the jejunal mucosa from the patients with untreated adult coeliac disease to liberate amino acids from the gluten peptides was significantly less than that of the mucosa from control subjects. 3. No significant difference from normal was found when jejunal mucosa from patients with treated adult coeliac disease was studied irrespective of whether or not the mucosa was histologically normal. 4. These results indicate that the impairment of jejunal mucosal digestion of gluten in untreated adult coeliac disease is a secondary phenomenon and do not support the hypothesis that coeliac disease is due to the absence from the intestinal mucosa of an enzyme normally concerned in the digestion of gluten.