  The neutron-proton differential flow is shown to be a very useful probe of
the isospin-dependence of the nuclear equation of state (EOS). This novel
approach utilizes constructively both the isospin fractionation and the nuclear
collective flow as well as their sensitivities to the isospin-dependence of the
nuclear EOS. It also avoids effectively uncertainties associated with other
dynamical ingredients of heavy-ion reactions at intermediate energies.
