An immuno responsive membrane was prepared by immobilizing Wasserman antigen to acetylcellulose. The antigen-binding membrane was used in developing an immunosensor for determining specifically corresponding antibody in serum. The senor is characterized by electrochemical determination of the antigen-antibody complex formed at the membranesoluti on interface. Experimental data on the applicability of the sensor to serology tests for syphilis are given as well as an outline of a possible mechanism for evaluating the electrical potential difference across the antigen-binding membrane.