The structure of the immobilized Langmuir-Blodgett films of IgG and their immunological activity were studied by means of ellipsometry. The dependence of the film thickness on the surface pressure of deposition provides evidence of the tilting of molecules with an increase in the pressure. Below pressures of 30 mN m-1 the thickness of the film is approximately 4 nm which coincides with the smallest dimension of the IgG molecules. At pressures between 30 and 40 mN m-1 the thickness increases sharply achieving a value of about 10 nm which is equal to the largest molecular dimension. A further increase of pressure does not show in terms of growth of the thickness. This means that the films are transferred from the water-air interface in the form of a 2-D ordered monomolecular layer. The dependence of the immunological activity on the pressure of deposition was shown to have a descending pattern. Different mechanisms are proposed which explain the decrease of the immunological activity of the IgG molecules in the film with an increase of the surface density such as the blocking of the active sites and the decrease of the conformation mobility of the Fab fragments.