Dabsyl-alginate, the dye-labeled substrate of alginate lyase, was prepared. Low-viscosity alginate, which was prepared by enzymatic degradation of sodium alginate, and tetramethylenediamine were conjugated by reductive amination. Then, dabsyl-Cl was coupled with the primary amino group of the aminobutyl-alginate. The assay for endo-alginate lyase using dabsyl-alginate was simple to use. The amount of dyed fragment released from the substrate increased with the amount of the enzyme.