To study the conditions under which supramolecular dynamic aggregates of proteins (clusters) are formed in protein solutions, the thermodynamic stability of the model system water-biopolymer-electrolyte was analyzed. The analytical expression of the coefficient of the stability of the system (partial derivativemu(1)/partial derivativem(2))(m3) was obtained, where mu(1) is a chemical potential of water and m(2) and m(3) are molar concentrations of the biopolymer and the electrolyte. The expression was used to obtain equations of spinodal and two quasi-spinodals that converge at one point in phase diagram. They correspond to the lines of critical and supercritical phase transitions (PTs) that give rise to the various structural-dynamic states of solution. These equations relate the critical composition of the system (m(2)/m(3))(cr) to the specific characteristics of the biopolymer and electrolyte ((m)2/(m)3)(cr)=f(v, z, Delta), where v is the number of the electrolyte ions that interact with the biopolymer, z is its charge, and A is the function of concentration-dependent variation in the coefficient of activity of the electrolyte. In the region of one of the quasi-spinodals, in a narrow electrolyte concentration range, the most favorable conditions for the formation of crystal line-phase nuclei have been created. The author presents experimental data obtained using the EPR spin-label method that provide evidence for PTs in solutions of serum albumin molecules which are in a qualitative agreements with phase diagram predictions. The mechanism of protein cluster formation is discussed. (C) 2004 Elsevier B.V. All rights reserved.