Adsorption layers of beta -casein formed at the interface between air and a buffer including various concentrations of guanidine hydrochloride (GuHCl) were studied by neutron reflectivity and by bubble tensiometry. A transition in the structure and in the properties of the adsorption layer seems to occur around a GuHCl concentration of 1.5 M. The data are interpreted assuming that the adsorbed protein molecules behave like multiblock copolymers with alternating hydrophilic and hydrophobic sequences. Below the transition, the hydrophilic coils and the hydrophobic two-dimensional blocks have a fractal dimension larger than that beyond the transition where they have the features of either two-dimensional or three-dimensional excluded volume coils. The effect of temperature on these phenomena indicates that they are not dominated by hydrophobic interactions. Thus, the attractions between amino acids which are broken by GuHCl might be hydrogen bonds which are frequently encountered in the secondary structure of polypeptide chains. 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 from a buffer devoid of denaturing agent.