There are many widely used techniques for fractionating the serum proteins. “Salting out” by differential chemical solubility yields rough separation into albumin and globulin. Cohn devised a more complicated chemical fractionation method by which certain parts of the protein spectrum may be separated from one another in a large-scale industrial-type procedure. Albumin is most often assayed by a chemical method (biuret) that reacts with nitrogen atoms, or with a dye (such as bromcresol green or bromcresol purple) that preferentially binds to albumin. The ultracentrifuge has been used to study some of the subgroups of the globulins. This is possible because the sedimentation rate at high speeds depends on the molecular size and shape, the type of solvent used to suspend the protein, and the force of centrifugation. The velocity of any particular class of globulins under standard conditions depends primarily on molecular size and is known as the “Svedberg number”; the most common classes of globulins are designated as 7S, 19S, and 22S. Electrophoresis separates molecules by the electrical charge of certain structural atomic configurations and is able to subdivide the globulins, but only into groups rather than into individual proteins. Serum protein nomenclature derived from electrophoresis subgroups the serum globulins into alpha, beta, and gamma, corresponding to electrophoretic mobility. Using antibodies against antigens on the protein molecule, immunoassay (including radial immunodiffusion, Laurell “rocket” electroimmunodiffusion, immunonephelometry, immunofluorometry, and radioimmunoassay, among others) is another technique for measuring serum proteins that is assuming great importance. Immunoassay techniques quantitate individual proteins rather than protein groups and in general produce reliable results with excellent sensitivity and specificity. Immunoelectrophoresis or similar techniques such as immunofixation goes one step beyond immunoassay and separates some of the individual globulin molecules into structural components or into subclasses. Immunoelectrophoresis also can detect abnormal proteins that either differ structurally from normal proteins or are produced with different proportions of structural components.