Intestinal absorption of mannitol (used as a marker of normal monomer passage) and intestinal absorption of lactulose (used as a marker of macromolecule exclusion) were measured in 15 healthy subjects and in 20 patients with food allergy. When studied in the fasting state, there was no apparent difference in the absorption of mannitol between control subjects and allergic patients although the absorption of lactulose was more marked, but not significantly, in the latter group. Ingestion of an allergen by these patients led to a significant increase in the absorption of lactulose (intestinal passage increased by a factor of two). Administration of 300 mg cromoglycate 1/4 hour before the provocation test completely averted abnormal intestinal permeability. Digestive absorption of cromoglycate was also studied in the same 15 control subjects and 20 allergic patients. The peak plasma concentration and area under the curve were significantly greater in healthy subjects when the study was carried out in the fasting state. The application of an oral provocation test with an allergen also significantly increased the plasma concentration of cromoglycate in allergic patients. Contact between an allergen and the digestive tract in allergic patients therefore leads to an increased intestinal absorption of macromolecules but also to cromoglycate passage, which inhibits allergic reactions.