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A method has been developed for applying the phase rule to systems of several protein components in serum. The globulin fractions which have been investigated appear to be homogeneous substances.

Changes were studied in the standard solubility curve of fresh serum proteins by alterations in pH, temperature, concentration of protein, and nature of the salt used for precipitation. The principal factor affecting the precipitation of protein fractions was a change in temperature. In order to investigate the proteins in their original states low temperatures are necessary. Protein fraction A is altered by a change in pH and with the use of (NH(4))(2)SO(4) as a precipitant, fraction B by a change in pH and temperature, and use of (NH(4))(2)SO(4), C by a change in temperature and concentration of the protein, and D by a change in temperature and pH. The solubility of D is independent of the amount of protein in solution in high concentrations of salt.

There are four different kinds of protein in blood serum as shown by the solubility curves. They must be either single proteins, several continuous series of compounds, or solid solutions. The solid protein phases are hydrated. There are definite sex and species differences.