On the basis of a variational principle a quantitative theory is developed enabling a thermodynamic description in terms of mean field approximation of heteropolymer mixtures of macromolecules with an arbitrary distribution for both degree of polymerization and composition. Rather simple general equations are derived to calculate compositions and volume fractions of spatially homogeneous macroscopic phases as well as to find the cloud-point curve, spinodal, and critical points. Potentialities of general theory are illustrated for copolymers synthesized by traditional methods.