The antisecretory factor (AF) is a 41-kDa protein that provides protection against diarrheal diseases and intestinal inflammation. Its cDNA has been cloned and sequenced. AF is highly potent, with 10(-12) mol of recombinant AF being sufficient to counteract experimentally induced diarrhea in rat. The antisecretory activity is exerted by a peptide located between positions 35 and 50 of the AF sequence. Synthetic peptides based on this sequence are promising candidates for drugs to counteract intestinal hypersecretion, as well as imbalances of fluid transport in other body compartments. AF probably exerts its effects via nerves; AF immediately and potently inhibits ion transport across isolated nerve membranes from Deiters' cells. Immunocytochemistry has shown that AF is present in most tissues in the body, and in situ nucleic acid hybridization has shown that cells that store AF are also capable of AF synthesis. The endogenous plasma level of AF is increased by enterotoxins and by certain food constituents such as hydrothermally processed cereals. These cereals significantly improve clinical performance in patients suffering from inflammatory bowel diseases. AF-enhancing food also protects domestic animals against diarrheal diseases, and such feed has been used successfully in Swedish swine farming for the past 10 years. Increased understanding of AF action might result in expanded clinical applications and confirm that AF is an important regulator of homeostasis.