Portions of jejunal biopsies from control subjects and from patients with coeliac disease were cultured for 24 hours using an in vitro organ culture technique. Alkaline phosphatase activity was measured in the tissue and medium before and after culture; enzyme activities were expressed per microgram tissue DNA. The increase in enzyme activity during the culture period was taken to represent net enzyme synthesis. Alkaline phosphatase synthesis by mucosa from patients with untreated gluten-sensitive coeliac disease and by mucosa from patients with non-responsive coeliac disease was significantly less than that by normal mucosa. Alkaline phosphatase synthesis by mucosa from patients with treated gluten-sensitive coeliac disease was greater than that by untreated coeliac mucosa but was less than normal mucosa. Sequential studies of alkaline phosphatase synthesis by jejunal mucosa from seven patients with coeliac disease, before and after successful treatment by gluten withdrawal, showed an increase in synthesis in all patients. Study, by analytical subcellular fractionation with sucrose density gradient centrifugation, of the properties of the organelles of cultured control tissue showed good preservation of their integrity. A striking finding, however, was the decrease in malate dehydrogenase with a corresponding marked increase in lactate dehydrogenase. This would be expected to be followed by a shift from aerobic to anaerobic metabolism. Analytical subcellular fractionation of cultured mucosa from patients with coeliac disease gave similar conclusions. There was, however, a marked improvement of the brush border abnormalities, characteristic of coeliac disease, during culture with increased enzyme activities and membrane equilibrium density in the sucrose gradients.