  We investigate an infinite dimensional partial differential equation of
Isaacs' type, which arises from a zero-sum differential game between two
masses. The evolution of the two masses is described by a controlled
transport/continuity equation, where the control is given by the vector
velocity field. Our study is set in the framework of the viscosity solutions
theory in Hilbert spaces, and we prove the uniqueness of the value functions as
solutions of the Isaacs equation.
