Electrochemiluminescent-labels and enzyme substrates, which preferably are conjugated, are used in immunoassays and electrochemiluminescence is generated catalytically. In conventional electrochemiluminescence immunoassays, an anti-analyte antibody molecule can give rise to typically 6-8 electrochemiluminescence-active ruthenium atoms, while in the present invention, each enzyme-labeled anti-analyte molecule can give rise to thousands of electrochemiluminescence-active ruthenium atoms per second. An exemplary immunoassay is based on a catalytic process employing .beta.-lactamase-conjugated anti-analytes which enzymatically hydrolyze electrochemiluminescent-labeled substrates, making them strongly electrochemiluminescent. The electrochemiluminescence signal generated by each anti-analyte molecule (i.e., each analyte molecule) is much greater than with the conventional method. Accordingly, greater sensitivity can be gained in the measurement of low concentrations of a given immunoassay analyte.