Surfactant protein C (SP-C) exists in alpha-helical and beta-sheet conformation. The influence of these conformations on the surface behavior of spread layers containing dipalmytoyl phosphatodylcholine and the surfactant protein B was investigated. The beta-sheet conformation of SP-C, which is formed during the preparation procedure at high lipid/protein concentrations, was confirmed by using circular dichroism (CD) and FTIR spectroscopy. Using a captive bubble device, the surface pressure/area isotherms, the surface dilatational elasticity and viscosity, and the main relaxation times of a transient stress relaxation were determined. Surface layers containing SP-C in beta-sheet have no protein squeeze-out plateau. The surface dilatational elasticity is increased, which leads to a film of low compressibility. The relaxation times were increased by more than one order of magnitude, and the decays after surface compression and dilatation did not approach each other. Therefore, the surface behavior of layers containing SP-C in beta-sheets remarkably differs from those containing SP-C in alpha-helical conformation. The presence of beta-SP-C prevents the film from meeting the function of a pulmonary surfactant layer.