A new and sensitive method for detecting immunologic reactions was described in two previous articles. This method, called immuno-electroadsorption or IEA, consists in depositing first a thin layer of antigen on a metalized glass slide under the influence of a weak current. The slide is then immersed in a dilute solution of the serum to be tested. A weak current is also used, but this time the polarity is reversed, the slide being connected to the anode. It is found that the layer adsorbed after serum treatment is thicker with a homologous immune serum than with a normal or a heterologous serum. The thickness of the adsorbed layer from a serum varies from 50 A to over 400 A depending upon the time of adsorption, the antigen, the dilution of the serum, and the species of animal tested. The presence of a weak current permits the detection of a specific immunologic reaction with a very crude antigen preparation or with a very dilute antiserum, or both.