Interfacial shear properties of adsorbed protein layers at the air/water interface were determined using a Couette-type surface shear rheometer. Such experiments are often used to determine a steady-state ratio between stress and rate of strain, which is then denoted as "surface shear viscosity". However, by measuring the stress on the protein layer as a function of time at a fixed shear rate, more information on the mechanical properties of the protein layers can be obtained. The development of the stress exerted on the inner disk with time was recorded. Initially the stress increased steadily with time, then it went through a maximum and next attained a steady-state value from which the surface shear viscosity is usually determined. Stress-strain curves can be calculated from the data obtained. Differences in the stress-strain curve were observed for the proteins studied (ovalbumin, beta-lactoglobulin, glycinin), and the shape of the stress-strain curve is discussed. The maximum in the stress-strain curve can be regarded as a kind of fracture/yield stress. This implies that the strain at the maximum is a fracture strain, and the decrease in stress reflects a kind of breakdown of the protein film structure.