Adsorption of lysozyme at the surface of a buffer solution at 25 degreesC, pH 7, and ionic strength 0.1 is studied under different denaturing conditions using on X-ray reflectometry technique. When the lysozyme is fully denatured with urea and dithiothreitol (DTT), its measured adsorption profile is very well explained by the scale law (z(-4/3)) profile theoretically predicted for polymer adsorption. When no denaturing agent is present, a monolayer is also produced, but the adsorption profile cannot be explained by a monolayer of nondenatured lysozyme; furthermore, it is close to the one obtained for lysozyme partially denatured with urea. A PMIRRAS study of native lysozyme adsorbed at the air-buffer interface shows that the secondary structure of the protein is modified: most of the alpha-helices are replaced by beta-sheets. In contrast, when the lysozyme is adsorbed below a monolayer of oleic acid at the air-buffer interface, that is, on a hydrophilic interface, the protein forms a monolayer whose thickness, 3.0 nm, is equal to one dimension of crystallized lysozyme. Under such conditions, the adsorbed protein is not denatured. Thus the hydrophobic nature of the air-water inter-face yields partial denaturation of the protein upon adsorption, but the disulfur bridges and beta-sheets prevent total denaturation. (C) 2003 Elsevier Inc. All rights reserved.