The complex formation of Bovine Serum Albumin (BSA) with anionic polyelectrolyte (polyacrylic acid, PAA) in aqueous solution was studied by a fluorescence technique, pH titration and HPLC analysis. The character of the interactions and solubility of the polycomplex particles depends on the BSA/PAA ratios and the pH of solution. The interaction at pH > pI (isoelectric point of BSA) (pH 6.0-7.0) is negligible weak and at pH 5.0 results with the formation of stable water-soluble polycomplexes at a wide range of protein/polymer ratios. The fluorescence intensity of BSA sharply decreased when an different amount of PAA was added and its maximum wavelength shifts towards the blue region. The protein molecules in the structure of soluble polycomplex particles are densely covered by the shelf of a polymer coil and practically "fenced off" from the water environment. This effect was reinforced by the increase of protein components. Existence of soluble and insoluble PAA-BSA complexes have been observed at pH < pI (pH 4.0-4.3). These soluble complexes characterized by the structure of particles in which protein molecules are densely covered by the shelf of a polymer coil. By the increase in the protein concentration, these complexes aggregate to an interpolymer species.