Crystallization of recombinant human growth hormone (HIGH) at elevated pressures was investigated in the presence of 6,000 molecular weight poly(ethylene glycol; PEG-6000). Crystallization of rhGH at atmospheric pressure occurred at a protein concentration of 15 mg/mL in 6% PEG-6000. Crystallization did not occur in the same solutions at 250 MPa. In contrast, at a pressure of 250 MPa in the presence of 8% PEG-6000, rhGH readily crystallized from solutions containing 35 mg/mL rhGH, whereas amorphous precipitate formed in the same solutions at atmospheric pressure. Osmotic virial coefficients were determined from static light scattering measurements and combined with a hard-sphere activity coefficient model to predict rhGH activity coefficients as a function of pressure and PEG concentration. Predicted activity coefficients quantitatively matched those determined from equilibrium solubility measurements. The ability to adjust the thermodynamic non-ideality with pressure provides a valuable tool to study protein crystallization in addition to providing a methodology for obtaining crystals at elevated pressures. Biotechnol. Bioeng. 2010;107: 663-672. (C) 2010 Wiley Periodicals, Inc.