We present a numerical simulation of the phase behavior of a simple model for a protein solution. We find that this system can occur in three phases, namely a dilute liquid, a dense liquid and a crystal. The transition from dilute-liquid to dense-liquid takes place in the regime where the fluid phase is metastable with respect to the crystal. We have computed the relative stabilities of different crystal morphologies. In addition, we have analyzed the "nucleation" of the native state of an isolated lattice protein. Using a "Go" model [N. Go, J. Stat. Phys. 30, 413 (1983)] to describe the protein, we show that a first order transition exists between the native and the coil state. We show this by analyzing the free energy barrier for the coil-to-native transition. (C) 2003 American Institute of Physics.