Progress in protein chemistry made during the ten years has resulted in the preparation of well-defined human plasma proteins and, consequently, of highly specific antiserums to the proteins. The most important contribution during recent years to quantitation of serum protein components has been development of the single diffusion type of precipitin reaction. The term "radial immunodiffusion" is applied to a system in which the antibody is incorporated in the agar and the gel spread out on a surface with the antigen diffusing radially, starting from a circular reservoir. The diameter of the precipitin zone formed is directly proportional to the amount of antigen present in the test serum, and inversely proportional to the concentration of antibody incorporated in the agar gel. Mancini contributed handsomely to this technique with kinetic studies of the system. By standardizing the technical conditions of the test, one can utilize this principle for immunochemical quantitation of