We report a molecular dynamics computation of the entropic depletion f
orce induced between two large spheres (colloidal particles) immersed
in a fluid of small spheres. The effective pair potential obtained by
numerical integration of the force is used in a Monte Carlo study of t
he phase behaviour of the binary mixture. The simulation results are c
ompared with the relevant theoretical predictions that follow from var
ious integral equations for liquid mixtures. The simulations provide e
vidence for a spinodal instability in a liquid mixture of hard spheres
with a size ratio of 0.1.