This work was targeted at characterizing the effect of high pressure on the kinetics of lysozyme crystallization via a series of experiments and mathematical modeling. Pressure effects on nucleation and crystal growth were distinguished by evaluating the rate constants for the two kinetic processes at various pressures;In the first phase of the work, the effect of high pressure on lysozyme crystallization was examined with a primary objective of increasing the rate of crystallization. Lysozyme crystallization was carried out initially at a concentration of 40 mg/mL, pH 4.5 and 0.8 M sodium chloride at pressures of 0.1, 50, 100 and 150 MPa. A factorial experiment was performed with level of pressure and time of pressurization as the two factors with four levels of each factor. Protein recovery in crystals at atmospheric pressure was significantly different from all the pressure-treated samples. An overall enhancement of the rate of crystallization could be obtained by a period of pressure followed by growth at atmospheric pressure. Initial duration at elevated pressure for either 10 min or 2 h followed by growth at atmospheric pressure for a total time of 36 h (including the time of pressurization) resulted in overall crystallization rates that were 2-3 times higher than those obtained without pressurization. Pressure effects were not evident for lower protein concentrations of 19 and 28 mg/mL. Enzymatic activity was unchanged and protein solubility appeared unchanged as a result of pressurization. A similar behavior was observed at a pH closer to the iso-electric point (pH 9.5), i.e. a period of pressurization followed by growth at atmospheric pressure gave crystalline protein recoveries that were 1.5 times higher than the crystallization at atmospheric pressure;The population balance model was used as a relatively easy but efficient way to obtain the kinetic constants for nucleation and crystal growth. Both nucleation and crystal growth rates were expressed as power law functions of supersaturation and the rate constants were evaluated by a combination of experimental observations and mathematical modeling at atmospheric pressure (0.1 MPa) and at three different pressures (34, 68 and 100 MPa). Single crystal growth measurements (atmospheric pressure only), experimental concentration profile and a count of the total number of crystals at the end of the batch time were sufficient to determine the kinetic expressions for nucleation and crystal growth. The model was successfully used to predict results at two other initial protein concentrations and showed that the kinetic constants for nucleation and growth were an order of magnitude lower at the three high pressures. Transition state theory was used to quantify the effect of pressure on the crystallization kinetics of lysozyme. The activation volume ([delta] V*) for nucleation and crystal growth were estimated to be +90 cm3/mol and +40 cm3/mol respectively, though the behavior did not conform well to this model.