A method, based on the liberation of a dye complex from Blue Dextran 2000, is described for the assay of dextranase. The reaction was conducted at 40 °C for 5 min at pH 4.0 in a mixture containing 0.5% Blue Dextran 2000 and the enzyme. The Km for the hydrolysis of Blue Dextran 2000 was estimated at 2.5 × 10−6 M.