A new technique is introduced to study the binding of drugs to macromolecules. The method utilizes rotatingdisk polarography in which a semipermeable membrane covers the surface of the electrode. The membrane permits passage of an electroactive drug but restricts permeability of the macromolecule and of bound drug. The current generated at the electrode at steady state is directly proportional to the concentration of free drug in bulk solution, from which a calculation of the amount of drug bound may be made. The methodology used provides a complete characterization of binding within about 1 hr. and is, therefore, faster than conventional techniques presently used. Another advantage of this technique is that relatively small amounts of the compounds being studied are required. The method is compared with equilibrium dialysis and ultrafiltration techniques by studying the binding of 2-(4'-hydroxybenzeneazo)benzoic acid (I) to serum albumin and is found to give precisely the same results. 4 4 - Chlorophenoxy)-a-methylpropionic acid (ll), in previous spectre photometric studies, showed an anomalous effect on the binding of I to rat serum albumin. At low concentrations, there was an indication that 11 may increase the binding of I to rat serum al-bumin. This phenomenon was reinvestigated using the new electrochemical method; it was found that 11 competitively inhibits the binding of I at all concentration levels of the inhibitor. The conclusion is reached that I1 affects the nature of the I binding site rather than the number of molecules bound. Keyphraees 0 2-(4'-Hydroxybenzeneazo)benzoic acid binding to