  The localization length for the center of mass motion of a matter lump,
induced by gravitation, is obtained, without using any phenomenological
constants. Its dependence from mass and volume is consistent both with unitary
evolution of microscopic particles and with the classical behavior of
macroscopic bodies required to account for wave function collapse in quantum
measurements, the transition between the two regimes being rather sharp. The
gravitational interaction of nonrelativistic matter is modelled by a Yukawa
Hamiltonian with vanishing pion mass, no gravitational background is needed and
the only hypothesis consists in assuming unentanglement between matter and the
Yukawa field.
