The driving force for crystallization is the difference in chemical potential between the protein in supersaturated solution and at its solubility. This chemical potential difference depends in a complicated manner on protein concentration and solution conditions (temperature, ionic strength and pH). Consequently, driving forces estimated from the nominal ideal solution supersaturation may be in error. In this work, the chemical potential of protein in solution is estimated from a virial expansion in protein concentration. The thermodynamic driving force for the crystallization of hen egg white lysozyme is calculated as a function of temperature, salt concentration, and protein concentration. In all cases examined here, the driving force for crystallization is lower than that for the corresponding ideal solution and the deviation from ideal solution behavior increases with the protein solubility.