A simple and rapid method for determining the cellobiase activity in purified enzyme preparations was developed. It is based on a series of consecutive enzymatic reactions, i. e. hydrolysis of cellobiose by cellobiase, oxidation of the forming glucose by glucose oxidase, and formation of a dyed product under peroxidase action in the same reaction system. The dyed product is recorded spectrophotometrically at 460 nm. One measurement takes from 2-3 to 7-10 min depending on a particular method of the activity determining. The reagent which is used for the activity determining can be obtained in the yophylized form and used repeatedly. The relative deviation of the method is 5-7%.