The application of high hydrostatic pressure is an effective tool to promote dissolution and refolding of protein from aggregates and inclusion bodies while minimizing reaggregation. In this study we explored the mechanism of high-pressure protein refolding by quantitatively assessing the magnitude of the protein-protein interactions both at atmospheric and elevated pressures for T4 lysozyme, in solutions containing various amounts of guanidinium hydrochloride. At atmospheric pressure, the protein-protein interactions are most attractive at moderate guanidinium hydrochloride concentrations (similar to 1-2 molar), as indicated by a minimum in B-22 values, In contrast, at a pressure of 1,000 bar no minimum in B-22 values is observed, indicating that high pressures colloidally stabilize protein against aggregation. Finally, experimental values of refractive index increments as a function of pressure indicate that at high pressures, wetting of the hydrophobic surfaces is favored, resulting in a reduction of the hydrophobic effect, This reduction in the hydrophobic effect reduces the driving force for aggregation of (partially) unfolded protein.