  We consider the Fundamental Plane of elliptical galaxies lensed by the
gravitational field of a massive deflector (typically, a cluster of galaxies).
We show that the Fundamental Plane relation provides a straightforward
measurement of the projected mass distribution of the lens with a typical
accuracy of ~0.15 in the dimensionless column density kappa. The proposed
technique breaks the mass-sheet degeneracy completely and is thus expected to
serve as an important complement to other lensing-based analyses. Moreover, its
ability to measure directly the mass distribution on the small pencil beams
that characterize the size of background galaxies may lead to crucial tests for
current scenarios of structure formation.
