  We have calculated the chameleon pressure between two parallel plates in the
presence of an intervening medium that affects the mass of the chameleon field.
As intuitively expected, the gas in the gap weakens the chameleon interaction
mechanism with a screening effect that increases with the plate separation and
with the density of the intervening medium. This phenomenon might open up new
directions in the search of chameleon particles with future long range Casimir
force experiments.
