  A rigorous microscopic calculation of the polarizability of disordered
mesoscopic particles within the grand canonical ensemble is given in terms of
the supersymmetry method. The phenomenological result of Gor'kov and Eliashberg
is confirmed. Thus the underlying assumptions of their method are justified.
This encourages application of RMT in the Gor'kov--Eliashberg style to more
complicated situations.
