Abstract
A differential equation model with a polynuclear growth mechanism was formulated for a theoretical understanding of protein crystallization. The model equation contains two parameters characterizing nucleation and growth: the number of protein molecules constituting a critical nucleus and the order of growth kinetics. This model was applied successfully to explain the experimental data on the protein concentration changes due to nucleation and crystal growth of tetragonal and orthorhombic hen egg-white lysozyme. It was shown that the critical nucleus most probably consists of three or four molecules. The range and extent of the validity of the present model and analysis are discussed.