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To determine if hydrophobic surfactant proteins affect the stability of pulmonary surfactant monolayers at an air/water interface, the studies reported here compared the kinetics of collapse for the complete set of lipids in calf surfactant with and without the proteins. Monomolecular films spread at the surface of captive bubbles were compressed at 37°C to surface pressures above 46mN/m, at which collapse first occurred. The rate of area-compression required to maintain a constant surface pressure was measured to directly determine the rate of collapse. For films with and without the proteins, higher surface pressures initially produced faster collapse, but the rates then reached a maximum and decreased to values <0.04min⁻¹ above 53mN/m. The maximum rate for the lipids with the proteins (1.22±0.28min⁻¹) was almost twice the value for the lipids alone (0.71±0.15min⁻¹). Because small increments in surface …