A procedure for the quantitative description of the simplest forms of self-organization of biological molecules at interfaces is proposed. The procedure uses the method of an inclined polarized ray, which makes it possible to determine the degree of the orientation order of molecular fragments and the width of the anisotropic surface layer. The quantitative estimates were obtained in the study of the microbial polysaccharide dextran using the methods of an inclined polarized ray and flow birefringence. It was shown that molecular fragments of dextran are spontaneously oriented predominantly parallel to the interface, and the thickness of oriented surface layers is macroscopic (0.14 mm).