  The mass of a scalar field mediating a fifth force is tightly constrained by
experiments. We show, however, that adding a quartic self-interaction for such
a scalar makes most tests much less constraining: the non-linear equation of
motion masks the coupling of the scalar to matter through the chameleon
mechanism. We discuss consequences for fifth force experiments. In particular,
we find that, with quartic coupling of order unity, a gravitational strength
interaction with matter is allowed by current constraints. We show that our
chameleon scalar field results in experimental signatures that could be
detected through modest improvements of current laboratory set-ups.
