DNA-stabilized Ag nanoclusters, AgNCs, act as fluorescent labels for probing enzyme activities and their substrates. The effective quenching of AgNCs by H(2)O(2) enables the probing of H(2)O(2)-generating oxidases. This is demonstrated by following the glucose oxidase-stimulated oxidation of glucose through the enzyme-catalyzed formation of H(2)O(2). Similarly, the effective quenching of the AgNCs by quinones enabled the detection of tyrosinase through the biocatalyzed oxidation of tyrosine, dopamine, or tyramine to the respective quinone products. The sensitive probing of biocatalytic processes by the AgNCs was further implemented to follow bienzyme catalytic cascades involving alkaline phosphatase/tyrosinase and acetylcholine esterase/choline oxidase. The characterization of the alkaline phosphatase/tyrosinase cascade enabled the ultrasensitive detection of alkaline phosphatase (5 × 10(-5) units/mL) and the detection of o-phospho-l-tyrosine that is an important intracellular promoter and control growth factor.