A theoretical description of spontaneous spin polarization in systems of hybridized electron states on donor impurities with low concentration in a conduction band of a crystal is developed. On the base of general equations for the energy of spin splitting of the electron spectrum, formulated within the framework of the Fermi-liquid approach, it is shown that in systems of such a type the spin ordering mechanism of conduction electrons, which is more efficient than the usually considered mechanism of an exchange interaction with magnetic impurities, takes place. The possible realization of spontaneous spin ordering of electrons in hybridized states is substantiated for the case of total polarization of the system within the model of Fermi-liquid constants. Possibilities of finding manifestations of spontaneous spin polarization of an electron impurity system in thermodynamic properties (heat capacity and elastic moduli) are discussed. Evidences of such manifestations are presented from an analysis of available experimental data.