This substrate is soluble bovine neck ligament elastin that is heavily labeled with FITC such that the conjugate's fluorescence is quenched. Upon digestion by elastase or other proteases, the fluorescence is recovered. Digestion products from the elastin substrate have absorption maxima at ~492 nm and fluorescence emission maxima at ~515 nm. Because the assay is continuous, kinetics data can be obtained easily. The elastin derivative is also digested by proteases other than elastase.