The glycoprotein cerato-ulmin, which shows surface activity at concentrations as low as 30 ng/mL, stabilizes air bubbles, both spheroidal and of unusual morphology, in aqueous and aqueous ethanol solutions. In response to stress on a cerato-ulmin thin film, perhaps a monolayer, at an air-water interface, cylindrical and fibrilliform air bubbles are generated. They may be manipulated by application of tension, pressure or temperature. The relationship of these observations to elm trees infected with the cerato-ulmin producing fungus Ceratocystis ulmi suggests a mechanism whereby exceedingly low concentrations of cerato-ulmin may be concentrated and delivered to intercellular openings, thereby blocking the vascular system of elm trees. The correctness of this mechanism as a major factor in any tree wilt disease remains to be tested.