Covalent modification of human IgG by fatty acid esters (C-g and C-16) of N-hydroxysuccinimide was carried out. Surface hydrophobicity measurements, using the fluorescent probe 8-anilino-1-naphthalenesulfonate, indicate an increase in the surface protein hydrophobicity with an increase in the number and in the length of the attached alkyl chains. The modified IgGs decrease surface tension at the air/solution interface more effectively than the native protein The values of the molecular cross-sectional areas (Delta A) estimated from the kinetic data are in the range of 100-300 Angstrom(2) and reflect the size of protein segments at the interface during the adsorption process. About 40-50% increase in the Delta A was observed upon attachment of the C-g groups to the native Igc. The lengthening of the bound alkyl chain from C-g to C-16 results in a further increase in this value. The influence of the overall IgG hydrophobicity and the length of the attached alkyl chain on the dimensions of the mobile protein segment at the surface are discussed.