We have studied the effect of monolayer structure on dilatational characteristics (surface dilatational modulus and its elastic and viscous components) of protein monolayers ( beta -casein, caseinate, and whey protein isolate (WPI) spread on the air-water interface, at 20 degreesC, and at pH 5 and 7. The stress response to compression-expansion sinusoidal deformation of the interface in a modified Wilhelmy-type trough with two oscillating barriers was measured as a function of deformation amplitude (within the range of 1-20% of the initial area), frequency (within the range of 1-300 mHz), and superficial density (within the range of 1-8 mg/m(2)). The same experimental device coupled with Brewster angle microscopy makes it possible to determine the structure, morphology, and relative reflectivity of the monolayer. The monolayer structure and, especially, the conditions at which the monolayer collapses determine the viscoelastic behavior of the monolayer and the linear response of the stress to area deformation. The nonlinear viscoelastic behavior of the interface has been associated with the protein monolayer collapse. It was found not only that the dilatational modulus is determined by the interactions between spread molecules (which depend on the surface density) but also that the structure of the proteins spread on the monolayer also plays an important role.