The present report describes a sensitive and reliable method for potentiometric determination of hepatitis B surface antigen in biological fluids, as an application of enzyme-linked immunoassay technique by a "sandwich" procedure. Specific antibodies are immobilized on a gelatine membrane which covers the sensor of an iodide-sensitive electrode. After immersion of the modified electrode in a dilute solution of antigen, the enzymatic activity is evaluated in a solution of peroxidase-labelled antibodies, in the presence of substrate and iodides. The electrode potential is a function of antigen concentration in the solution. This new procedure should find its application in the determination of substances present in very low concentration in biological fluids.