Adsorption onto silica of native human IgG and its hydrophobized forms prepared by covalent attachment of 11, 25, and 52 C-8 alkyl chains was studied. All hydrophobized IgGs spontaneously form micellelike aggregates (nanoclusters) in aqueous solutions with a mean diameter of 40 +/- 2 nm. Adsorption isotherms are of a high affinity type. The plateau surface concentration of the isotherms depends on the degree of the protein modification, increasing for 11C(8)-IgG and 25C(8)-IgG and decreasing for 52C(8)-IgG as compared with the native protein. The isothermal enthalpies of adsorption for the native and modified IgGs at all degrees of silica surface coverage were found to be endothermic, that is, the adsorption process is entropically driven. For the native IgG, the adsorption isotherm is apparently reversible, while the isotherms for the modified forms display distinct hysteresis. The biological (immunological) activity of the desorbed molecules was evaluated, and it was found that all forms of IgG which were desorbed from silica display reduced ability to react with a specific antibody, goat antihuman IgG, compared to the corresponding form. before adsorption. The immunoassay on desorbed IgGs indicated that hydrophobic modification of the molecule reduced structural alterations observed on adsorption of the native IgG. The decrease in activity was much less pronounced in the range of surface coverage close to the plateau values. Possible mechanisms of adsorption of the hydrophobized forms of IgG are discussed.