  The onset of viscous fingering in the presence of a non monotonic viscosity
profile is investigated theoretically for two immiscible fluids. Classical
fluid dynamics predicts that no unstable behavior may be observed when a
viscous fluid pushes a less viscous one in a Hele-Shaw cell. Here, we show that
the presence of a viscosity gradient at the interface between both fluids
destabilize the interface facilitating the spread of the perturbation. The
influence of the viscosity gradient on the dispersion relation is analyzed.
