The adhesiveness of bronchial mucus is directly related to its role as a filter. In disease the increased tackiness of mucus may also play a role in difficulty of reopening closed airways. A study was made of the pressures required to reopen collapsed tubes whose walls were coated with mucus or similar polymer solutions. Opening pressures were recorded as a function of collapsing force, time of application of force, and length of joint ruptured. The rupture properties depend primarily on the viscosity of the fluid. The elastic properties may be important if reopening pressure is applied rapidly, as in percussion. Reopening pressure is dependent on time of collapse only for a short initial time, implying a critical collapse time in the lung after which it is much more difficult to reopen an airway.