The hydrophobic nature [Am. J. Physiol. 244 (Gastrointest. Liver Physiol. 7): G561-G568, 1983] of the mucosal lining of canine oxyntic tissue has been characterized by rinsing with saline or a phospholipid solvent (hexane) and by measuring the maximum contact angles as droplets of a hydrophilic liquid (saline) and a hydrophobic liquid (olive oil) advance over that surface. The results are consistent with the luminal lining as an adsorbed monolayer of surfactant whose outer shell of packed fatty acid chains resemble polyethylene. Phospholipid (dipalmitoyl lecithin) monolayers deposited in vitro and polyethylene were both found to be readily wetted by stomach mucus, reducing advancing contact angles (theta) by 74 and 83%, respectively, while eliminating altogether the minimum contact angle for receding saline. Taking theta values for "dry" tissue as the most conservative, the reduction in surface energy imparted by mucus was estimated as 30.3 ergs X cm-2, which dispels the objection that, on thermodynamic grounds, surfactant must form a bilayer rather than a monolayer. Thus, as a powerful wetting agent, mucus would play an important role in stabilizing and replenishing the absorbed surfactant monolayer proposed as the physical basis for the gastric mucosal barrier.