Abstract: Fraction I protein from plant leaves is purified and subsequently crystallized. The crystallization methods disclosed herein unexpectedly produce crystallization in all crop leaves examined, although for some species modification by salt addition is required to achieve crystallization and to prevent formation of substantial percentages of amorphous protein precipitates. It has been found that a fraction I protein solution, when mixed with a precipitant solution having a pH generally within the range of 4.8-7.2, in an amount and at a pH sufficient to provide a mixed solution (protein solution mixed with precipitant solution) having a final pH in the range of 6.6-7.0, causes crystallization of fraction I protein from plant leaves, provided that the precipitant solution is at a pH lower than the pH of the protein solution. Optimum results have been obtained when the pH of the precipitant solution is in the range of 5.0 to 6.0 and the protein solution in the range of 7.0 to 7.5.