IN the course of sedimentation velocity studies with serum protein mixtures, it has been noted from time to time that the concentrations of the different molecules calculated from the sedimentation diagram differ from those determined analytically1. The substance sedimenting at the higher rate decreases in amount to form units of a lower apparent sedimentation constant, even though it has appeared to be perfectly stable under the same conditions when it is present alone. It has been concluded that these changes are caused by some medium effect produced by the action of one protein upon the other. It was then reasoned that it should be possible to produce this effect by means of such lower molecular substances as amides, amino-acids, polypeptides, and protamines. The action of such dissociating compounds is found to be highly specific. Thus an amino-acid which acts strongly upon a certain protein may have no effect upon another, and vice versa, and the effect may depend upon the presence or absence of salt2,3.