  Starting from a protein solution which is metastable with respect to the
crystalline phase, the effect of adding semidilute non-adsorbing polymer is
considered. It is found to increase the chemical potential of the protein by a
few tenths of kT, which may be enough to lower the barrier to nucleation of the
crystalline phase by enough to allow crystallisation. It is also shown that
assuming that the polymer induces a pairwise additive attraction leads to
qualitatively incorrect results.
