Alpha-1-antitrypsin (α-1-AT) is a 52 kDa glycoprotein, which is produced by the liver, intestinal macrophages, monocytes and mucous membrane cells of the gut. It belongs to the group of acute phase proteins and is one of the most important proteinase inhibitor. α-1-AT inhibits, beside others, the proteinases trypsin and the elastase of neutrophiles. A lack of α-1-AT leads to an enhanced proteolysis. Only a very small amount of α-1-AT is cleaved or resorbed in the gut. Therefore the measurement of α-1-AT in stool reflects the permeability of the gut during inflammatory processes.