  Classical nucleation theory is used to estimate the free-energy barrier to
nucleation of the solid phase of particles interacting via a potential which
has a short-ranged attraction. Due to the high interfacial tension between the
fluid and solid phases, this barrier is very large, much larger than in hard
spheres. It is divergent in the limit that the range of the attraction tends to
zero. We predict an upper limit on nucleation in good agreement with the
results of experiments on the crystallisation of proteins.
