The rheological behavior of adsorption layers of beta-casein formed at the air/water interface was studied by bubble tensiometry. It was observed that in some instances, the dilational. modulus, epsilon, has only an elastic component while in other cases it has both an elastic and a viscous component. The conditions where the elastic modulus is significant have been defined. In these conditions, the measured dilational modulus is practically equal to a physical quantity measured in equilibrium conditions. Until a surface pressure, pi, of about 5 mN/m, it is proportional to the surface pressure. Referring to a multiblock polymer model, the proportionality coefficient gamma (epsilon = gammapi) is directly related to the fractal dimension of the polymer forming a pancake at the interface. An increase of temperature from 10 to 60 degreesC results in a decrease of gamma from 6.6 to 3.3, indicating that in these conditions the two-dimensional polymer changes from a partly collapsed structure to that of a two-dimensional self-avoiding walk. These results show that even with the flexible polypeptide chain of beta-casein, interactions between amino acids contribute significantly to the structure of the adsorption layer formed at the air-aqueous buffer interface at room temperature.