The purpose of the present work is to characterize the cathodic electrochemiluminescence of luminol with a view to its application in continuous immunoassay. The authors designed a method for continuous antibody determination from the change in electrochemiluminescent intensity of luminol resulted from antigen-antibody reactions. The cathodic electrochemiluminescences of luminol-labeled antigens in hydrogen peroxide solution were enhanced by the presence of the respective antibodies. Cathodic electrochemiluminescence Of a luminol-labeled antibody was also enhanced by the presence of antigen. The enhancement of electrochemiluminescence may be useful in homogeneous immunoassay of antibodies or antigens. Analysis of a batchwise reaction suggested that the presence of antibody increases the quantum yield of electrochemiluminescence of luminol-labeled antigen. And analysis of the electrolytic current of luminol and hydrogen peroxide at indium-tin oxide (ITO) cathode revealed that hydrogen peroxide was electrochemically active at the cathode, while luminol was inert. These results suggest that cathodic electrochemiluminescence of luminol or luminol-labeled proteins is triggered by hydroxide radical produced from hydrogen peroxide at the ITO cathode.