1. The conditions under which the phase rule may be applied to systems containing proteins are formulated.
2. An attempt was made to fractionate chymotrypsinogen, by crystallization in stages with increasing concentration of magnesium sulfate. No significant fractionation of the protein was achieved, but a small amount of impurity which affects the solubility, while having little influence on other properties of the material, was concentrated in the fractions first precipitated.
3. The solubility of the final fraction was independent of the amount of the saturating solid, from the first appearance of a solid phase, in solvents of three different pH's. The solubility was independent of the environment in which the crystals were formed (within the limits in which crystallization can be carried out) and the same value was reached from the supersaturated as from the undersaturated side. This material, therefore, conforms closely with the phase rule criteria of a pure protein.
